Federer dropped just two sets in winning last year's US Open

Federer struggled to get to grips with 6ft 9in Isner's serve and the American snatched the first set on a tie-break.

But the Swiss top seed broke immediately in the second set and from then on looked in control.

Federer went on to wrap up a 6-7 (4-7) 6-2 6-4 6-2 win and a fourth-round meeting with Felicano Lopez.

"It was really tough. His potential is good," said Federer of his encounter with Isner.

"He's always going to be tough to beat because of that serve. That serve is not going to go away any time soon."

Isner, 22, fired 18 aces but was undone by 42 unforced errors but he said: "Sitting down after the first set with the crowd going crazy. That was pretty neat and something I'll always remember.

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"I take away a great deal from this match. Just to think that four months ago I was unranked, to go from that to beating Roger Federer in a set, it's pretty cool."

Former champion Andy Roddick eased into the fourth round of the US Open with a 6-3 6-2 6-0 win over Thomas Johansson.

Roddick broke once in the first set, twice in the second and three times in the third to wrap up an easy victory over the ex-Australian Open champion.

�Fifth seed Roddick will next play Czech ninth seed Tomas Berdych, who beat Spain's Fernando Verdasco 7-6 6-1 7-6.

"We both serve big and hit a pretty big ball from the baseline," Roddick said.

"It's going to come down to a couple points here and there, whoever converts their chances on the other guy's serve."

Fourth seed Nikolay Davydenko became the first man to reach the fourth round at this year's US Open.

The Russian beat 28th seed Nicolas Almagro of Spain 7-5 6-0 7-5 to set up a meeting with Hyung-Taik Lee, who upset 19th seed Andy Murray.

Davydenko, who reached the semi-finals at last year's US Open had no fewer than 22 chances to break Almagro during their third-round encounter.

Tenth seed Tommy Haas made hard work of French veteran Sebastien Grosjean to reach the fourth round.

The German won the first two sets before Grosjean came back to level the match.

However, former world number two Haas broke at 4-3 in the decider and served out for victory.