Federer dropped his first set of the tournament against Blake

The top seed failed to serve out the match in the third set and missed a match point in the tie-break as he lost his first set of the tournament.

Federer broke twice in the fourth but needed another two opportunities to close out the match on serve.

The defending champion will play Nikolay Davydenko in Saturday's semis.

News conference: Roger Federer

"The score says it all," said Federer. "The match could have been easier, it could have been tougher.

"We both missed our opportunities here and there but that is because we took our chances on our shots.

"It turned out to be a thriller."

In a pulsating encounter between two of the biggest hitters in the game, Federer produced some of his best and worst tennis.

I'd like to think it wasn't just a walk in the park for him James Blake

He played wonderfully to break serve in game nine but followed it with a poor effort when serving at 5-4.

A tie-break followed, in which Federer led 4-1, trailed 6-4 and saved three set points before converting his first chance thanks to a Blake net cord.

The next eight games illustrated why Federer is already considered one of the all-time greats - extraordinary winners from every angle left Blake reeling.

When the world number one broke for a 5-3 lead in the third set, the match appeared as good as over, but Federer is not rock solid when it comes to serving out.

With the pressure off, Blake let fly and broke back before forcing another tie-break.

Again, the American led 6-4 and, again, he let the chance slip away. Federer earned a match point only to watch as Blake bludgeoned a backhand winner past him.

It was enthralling stuff and the American crowd got what they wanted when Blake took his fifth set point.

Federer then fell 0-40 down in his first service game of the fourth set but it seemed to galvanise him and he hit back to hold serve and take a 5-1 lead.

Still it was not enough, as Blake broke back once and moved to 15-40 up when Federer served for the match for the third time.

Once again, the two-time champion stepped up a gear when the pressure was really on and after two hours 47 minutes Blake netted a backhand to give the Swiss victory.

"I'd like to think it wasn't just a walk in the park for him," Blake said afterwards.

"I take pride in the way I fought. It's a good feeling to know that I'm close to Roger. I guess he's human."