1720: From a technical perspective, Del Porto's victory over Verdasco was no classic but it probably gave us the most nail-biting conclusion of the 2009 ATP World Tour Finals so far. Next up is the one we've all been waiting for. It's Roger Federer against Andy Murray and it's live on BBC Three, BBC Radio 5 live sports extra and the BBC Sport website. The atmosphere is going to be out of this world. I honestly cannot wait. See you then.

1705: For Federer things are more complicated... if he overcomes Murray he'll be in a strong position to reach the last four but his spot will not be absolutely certain. If he loses to Murray, there will be a straight battle between Federer and Del Potro on Thursday - with the winner joining Murray in the semi-finals. A Murray win against Federer will send Verdasco out but if the Swiss and the Spaniard beat Murray, Verdasco could yet go through. Confused?

BBC Sport's Piers Newbery on Twitter: "Tie-break a bit of a damp squib but another good match. A win tonight puts Murray through. Pondering this over 1st mince pie of the season."

1700: Group A is getting rather tasty. Roger Federer leads the way from Andy Murray courtesy of a superior games won-to-lost record. The pair do battle in Tuesday's eagerly-anticipated evening match and if Murray wins he will be guaranteed a place in the semi-finals.

Del Potro 6-4 3-6 7-6 (7-1) Verdasco
Del Potro gets the tie-break started with a booming ace and swiftly secures the mini-break to open up daylight. Verdasco's forehand is badly malfunctioning and he fires long to fall 5-1 behind. The Spaniard natters to his support team but how are they going to help you now? Another mini-break puts Del Potro 6-1 up and Verdasco has basically given up. Del Potro serves out in style and his campaign is now very much alive. For Verdasco, things look bleak. Game, set and match Del Potro.

Del Potro 6-4 3-6 6-6 Verdasco
Ooh, the tension. Neither player dares to open their shoulders for fear of messing up. Verdasco edges to the hold of serve but does so in far from convincing fashion. We're into the tournament's first match-deciding tie-break.

BBC Sport's Piers Newbery on Twitter: "Forehand down the line or up the line? I'm a down the line man. Either way, delpo hits that one beautifully. Great end to this match..."

Del Potro 6-4 3-6 6-5 Verdasco*
Del Potro comes up with some of his most ruthless tennis of the match just when it matters most. First class service game from the big Argentine.

knowledge_is_good on 606: "Sublime tennis from DP as he comes back from 0-40 with some help from Nando, but still can't close it."

*Del Potro 6-4 3-6 5-5 Verdasco
At 5-2, I told you Del Potro's win was a matter of minutes away. I officially grant you permission to berate me in any way you like. Verdasco saves a match point and then holds serve when his opponent nets. I'll probably be wrong - again - but I still think Del Potro's going to win.

Del Potro 6-4 3-6 5-4 Verdasco*
Del Potro offers a masterclass in cracking under pressure. "Vamos," yells Verdasco after breaking his opponent's serve and keeping his campaign alive. Woeful from the Argentine, brilliant from Verdasco. The tournament could be set for its first deciding tie-break.

*Del Potro 6-4 3-6 5-3 Verdasco
It's difficult to be overly critical of Verdasco today because, aside from a sluggish start, he's done precious little wrong. But when Del Potro really wants it, there are few on the men's tour who can live with him. He bludgeons his way to 0-30 and, although Verdasco bounces back, the Argentine engineers a match point. The Spaniard saves that and the game so Del Potro will have to serve out the match. Will he? Won't he? We'll know in not too long at all.

Del Potro 6-4 3-6 5-2 Verdasco*
Simple hold for Del Potro. He's buzzing out there now and his win is now a matter of minutes away.

*Del Potro 6-4 3-6 4-2 Verdasco
I would hate to boast about my psychic abilities, but I did feel Del Potro was about to up the ante and, well, he did. After racing into a 0-40 lead when Verdasco goes wide and then nets, Del Potro lets his Spanish opponent back in. At 30-40, Del Potro leaves a forehand but it is called in. The world number five challenges the decision with a worried expression on his face. But Hawk-Eye shows the ball did go long and Del Potro has broken. He pumps his arms in celebration. That's the most emotion we're seen from Del Potro all day and victory is now within touching distance.

Bonzo on 606: "Pleased to see Verdasco make a fight of his matches. Was afraid he might get embarrassed by the bigger hitters."

Del Potro 6-4 3-6 5-2 Verdasco*
An immense forehand off the back foot gets an out-of-sorts Del Potro over the line. The game is a tale of Verdasco's limitations rather than Del Potro's excellence. David Felgate on BBC Radio 5 live sports extra describes the match as "patchy". He's right. You do, however, get the feeling that Del Potro could be about to turn the screw. But who knows?

Del Potro 6-4 3-6 3-2 Verdasco*
An immense forehand off the back foot gets an out-of-sorts Del Potro over the line. The game is a tale of Verdasco's limitations rather than Del Potro's excellence. David Felgate on BBC Radio 5 live sports extra describes the match as "patchy". He's right. You do, however, get the feeling that Del Potro could be about to turn the screw. But who knows?

*Del Potro 6-4 3-6 2-2 Verdasco
Nervy times for Verdasco as Del Potro brings up 30-30. But the Argentine berates himself after looping a backhand wide to give Verdasco game point and the Spaniard shows the Argentine exactly what he should have done with a backhand winner moments later. The crowd loved that one.

Del Potro 6-4 3-6 2-1 Verdasco*
Much better body language from Del Potro. Commanding hold. Boy this match is well poised.

*Del Potro 6-4 3-6 1-1 Verdasco
The depth of Verdasco's groundstrokes is pushing Del Potro further and further back, which is resulting in more and more unforced errors. The hulking Argentine, who has three titles to Verdasco's one this season, shows occasional flashes of brilliance but that will not be enough for him to beat the Spaniard. Verdasco battles his way to another important hold as David Law on BBC Radio 5 live sports extra reports an attendance of about 15-16,000. Apparently there are still tickets remaining for Federer-Murray tonight.

Del Potro 6-4 3-6 1-0 Verdasco*
Normal service resumed. Perhaps it dawns on Del Potro that his place in the tournament is hanging by a thread. He storms through the opening game of the deciding set and the pressure switches to Verdasco. David Felgate on BBC Radio 5 live sports extra believes Del Potro may be a player who looks to peak in time for the Grand Slams while not necessarily taking too much notice of his results in the other tournament. Would you agree?

Tricia in Glasgow via text on 81111
"Hoping Del Potro wins. Can't see Verdasco putting up much of a fight against Murray in their match on Thursday if he has already lost his first two matches and has no hope of going through."

*Del Potro 6-4 3-6 Verdasco
Goodness me, what is Del Potro playing at? The 'Giant of the Pampas' has dropped his head, he sulking and feeling sorry for himself. Verdasco holds a wonderful service game to love and will carry the momentum into a deciding set. Did anyone see that coming?

Del Potro 6-4 3-5 Verdasco*
This time Del Potro is forced to dig himself out of trouble early on after lashing a backhand wide to fall 0-30 down. But the Argentine can't see out the game and, from nowhere, Verdasco has a break point. A sloppy Del Potro goes way long and Verdasco has the break. The world number five won't want this to go to a deciding set so he'll have to buck his ideas up. Immediately.

Allie in Cardiff via text on 81111
"Re Sami's text, I must admit the late nights are irksome for those staying to watch, but the tournament must also cater for the American and Asian audiences, hence the later start and finish times!"

*Del Potro 6-4 3-4 Verdasco
It looks as though Del Potro is ready to make his move when he goes 0-30 up on the Verdasco serve. But the left-hander from Madrid comes up with a 131mph ace down the 'T' to get himself back in the game and then gets the better of his Argentine opponent in an exchange at the net.

Del Potro 6-4 3-3 Verdasco*
Life has rarely been so good for Del Potro on serve. So much so that his latest service game came and went in the blink of an eye. Take my word for it.

JK via text on 81111
"Murray made a comment about the fact he could win two matches and still not go through to the semis. Can you explain that one."
If Murray wins two matches but two other players in Group A do the same but drop fewer sets, he could go out. I think.

*Del Potro 6-4 2-3 Verdasco
Neither player appears to have the most detailed tactical plan. Del Potro is succeeding in taking the cover off the ball with almost every shot, while Verdasco is chasing his socks off before coming up with the odd gem of a winner. The Spaniard holds but surely needs to look for a break of serve soon? I'm not going to lie, this match is far from enthralling and the crowd know it.

BBC Sport's Caroline Cheese on Twitter: "Verdasco unable to handle the DelPo serve at the mo. To be fair, the Argentine does, in tennis parlance, serve out of a tree."

Del Potro 6-4 2-2 Verdasco*
It's Del Potro's turn to go 40-0 up this time and then, attempting an audacious forehand winner up the line, give his opponent a cheap point. At 40-15 he comes in to volley but Verdasco can only find the net.

bennie007 on 606: "For this match, logically my head says Del Potro. Verdasco's got two clay titles and one hard title and Popcorn's five hard titles and two clay titles. So I'm going to go for Popcorn."

*Del Potro 6-4 1-2 Verdasco
After racing 40-0 up, Verdasco lets Del Potro edge his way back into the game with a tempo lapse in concentration. But the 26-year-old keeps his neck in front with a drive-volley winner. Nice.

Chris n via text on 81111
"Re Sami's txt - I saw the entire Djokovic match, got the Thames Clipper back to Waterloo which took 30 mins and the last train to Woking at 1.05 am! To be fair, a message went round the arena mentioning the last train times, which were 15 mins after the match ended and the clipper always leaves 15 mins after the event finishes, whenever that is."

Del Potro 6-4 1-1 Verdasco*
Verdasco may be improving but, aside from game 10 of the first set, Del Potro is making fairly serene progress. Running round to take as many shots as possible on his forehand, the US Open champion sends a whippy smash rushing past his opponent to wrap up the game.

Freddie via text on 81111
"Would anyone who has tickets to the final like to take me with them? I'm psyched for a Federer Djokovic clash on Sunday. Loving the commentary."

*Del Potro 6-4 0-1 Verdasco
There's a spring in the step of Verdasco right now and the Spaniard's game is coming together at last. His shot selection and accuracy are particularly impressive but it is his serve that sees him through when Del Potro puts a backhand return into the net.

Tricia in Glasgow via text on 81111
"Have tickets for all day Friday. Can't wait! O2 looks fab! Sadly won't get to see Murray but hopefully by then he'll be resting for the semis on Saturday! C'mon Murray!"

Del Potro 6-4 Verdasco*
Picking the perfect moment to up the pressure on the Del Potro serve, Verdasco goes for the jugular. He chases from side to side, forward and back. His footwork and timing is much-improved. Del Potro brings up set point but nets and then, after a going long with a forehand, faces break-back point. It is saved with a monster serve but Verdasco wrong-foots him with a driven forehand winner to come within a point of the game once more. Del Potro looks tight as you like by manages to save again before flicking a forehand winner and taking the first set when left-hander Verdasco puts a backhand into the tramlines.

Sami via text on 81111
"I would have to say that no matter how great the opening singles matches have been, I am actually quietly quite relieved that I hadn't forked out the £50 to go to the evening sessions - especially last night! I would have been pretty narked off having to miss the end of such a scintillating match-up just so I could catch my last train. I can't believe the organisers have been so naive not to realise/factor in that the top players' matches are likely to go the distance and, therefore, jeopardise the likelihood of the spectators being able to stay for the entire match!"

*Del Potro 5-4 Verdasco
After the fireworks of Robin Soderling's shock victory over Rafael Nadal and Novak Djokovic's come-from-behind win against Nikolay Davydenko on day two, this match is not exactly setting the world alight. And that's putting it politely. Verdasco finds some rhythm to hold serve and Del Potro will have to serve out the set.

Del Potro 5-3 Verdasco*
Verdasco needs to get a grip with Del Potro's serve. He completely misreads a couple of thunderbolts sent down by the 'Tower from Tandil' and the 21-year-old Argentine is now a break away from a much-deserved one-set lead.

BBC Sport's Caroline Cheese on Twitter: "DelPo winning the battle of the forehands. Verdasco not quite on it yet. Must win for both players."

*Del Potro 4-3 Verdasco
A decent set of serves keep Verdasco's games for column ticking over. Before entering the court, the Spaniard could be seen charging up and down the corridor outside the dressing rooms. He seems to have knackered himself out because he's not doing too much running out there.

Del Potro 4-2 Verdasco*
Del Potro decides to bring out the big gun on game point. By big gun I mean his laser-guided missile of a forehand. It wins him the game. Gasps from the crowd as if to say, 'how on earth can he hit the ball that hard?'. He's not the US Open champion for no reason.

Debs via text on 81111
"I'm going for the evening match later- do you know if they'd be selling the radio-earpieces, so we can listen to 5live commentary?"
Debs, I have no idea but I will ask our very own Caroline Cheese, who's down at the O2 Arena today.

*Del Potro 3-2 Verdasco
Verdasco focused on keeping the ball in the court - easier said than done judging by the way he's played so far - and it pays dividends. The Spaniard holds but you get the feeling Del Potro is still playing within himself and has the ability to break serve almost at will.

Del Potro 3-1 Verdasco*
Tim Henman's former coach David Felgate in the 5 live sports extra commentary box and he's already contemplating a Federer-Del Potro showdown on Thursday for the right to join Andy Murray in the semi-finals. Wishful thinking. Del Potro holds when, with the Argentine stranded at the net, Verdasco goes long with a relatively easy lob.

*Del Potro 2-1 Verdasco
We have a break of serve and it arrived ridiculously easily for Del Potro. A horribly miscued forehand see Verdasco facing two break points at 15-40. Del Potro fires a thumping forehand in the Spaniard's direction and he shanks the return. Verdasco needs to step his game up. Del Potro has barely broken sweat but he's already in firm control of the first set.

Jamie Murray on Twitter: "Showcasing my footballing talents at wembley tomoro to support the help for heroes cause! Loving it!"

Del Potro 1-1 Verdasco*
As mentioned, these two have never previously met on the ATP Tour and they seem to be using the first couple of games to feel each other out. A straightforward hold puts Del Potro on the scoreboard.

*Del Potro 0-1 Verdasco
Confident start from world number eight Verdasco. Wearing an orangy-yellow top and white baseball cap, the Spaniard comes out of the blocks aggressively and hits a couple of running winners before Del Potro goes into the tramlines to concede the opening game.
* Denotes next to serve.

1431: Here we go!

1430: Verdasco, 28, gave world number of Federer a huge scare on Sunday by storming into a one-set lead and pushing the great Swiss all the way in the second. He faded in the third but will be no pushover for world number five Del Potro. The Spaniard has won the toss and will serve first.

1425: Del Potro has only recently recovered from a wrist injury and put in a below-par performance against Murray on Sunday. After dropping the first set, he showed a glimpse of his capabilities in the second but ran out of steam in the third. The 21-year-old US Open champion has the game's most venomous forehand, as Verdasco is about to discover.

1422: A little heads up for later. Murray-Federer will not start before 2045 GMT and you can watch the match on BBC 3, listen on 5 live sports extra and read game-by-game updates here on the BBC Sport website.

1418: As the players receive their boxing style introductions to the O2 Arena crowd, this is my chance to ask you to get involved in the chat. You can do so by text to 81111,606 or Twitter

1415: Wherever you are in the world, be sure to tune in to the commentary of the match on BBC Radio 5 live sports extra, which you can access through this website. Why not listen and read at the same time.

1412: Remarkably for two players within the world's top 10, Del Potro and Verdasco have never previously met on the ATP Tour. Their only career meeting, in a challenger event on the Segovia hard courts in 2006, saw Argentina's Del Potro come from a set down to win in three.

1410: Both Del Potro and Verdasco made losing starts to their Group A campaigns on Monday, going down to Murray and Federer respectively. The loser might as well book his flight home. That's how big this one is.

1405: It's the one you've all been waiting for. And it gets under way in about nine hours time. So while you're waiting for what promises to be an absolute belter between Andy Murray and Roger Federer, how do you like the sound of Juan Martin del Potro against Fernando Verdasco?