Djokovic has never reached the fourth round before

The Serb struggled with his form against the stubborn Stepanek and also needed a couple of sessions of treatment on his legs and his back.

But after four hours and 45 minutes on court, he finally came through 6-7 (4-7) 7-6 (7-5) 5-7 7-5 7-6 (7-2).

"It was a fantastic match," he said. "Unforgettable. I'm totally exhausted."

News conference: Novak Djokovic

Djokovic said he was shocked he had come through the match, despite being below his best.

"I'm very proud of myself. I don't even know how I managed to win. I was not on top of my game," he said.

"I'm happy with the way I was fighting all the way through. We had an unbelievable fight in a remarkable match. It was great fun."

Stepanek, who frequently interacted with a packed crowd in Louis Armstrong Stadium, crossed the net to hug his opponent after Djokovic sealed victory with a backhand winner.

"That match was really special. After such a match, it had to be done. I didn't plan it. I just felt it was something to do," said the 28-year-old.

"To play in such an atmosphere is just amazing. You want to leave everything you have on the court. I think we both did. I gave everything I had."

Djokovic now meets Argentine teenager Juan Martin del Potro, who upset 31st Jurgen Melzer in straight sets.

Second seed Rafael Nadal had a relatively easy passage into the third round of the US Open as Janko Tipsarevic retired from their match.

The Serb had treatment on a side injury but with Nadal leading 6-2 6-3 3-2, he decided he was unable to continue.

Nadal, who himself has been struggling with knee tendonitis, will face France's Jo-Wilfried Tsonga, who beat Tim Henman.

Ex-champion Marat Safin was seen off in straight sets by Stanislas Wawrinka.

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Russian Safin, winner in 2000, had four chances to break in the first set but could not take any of them, allowing Swiss player Wawrinka to take the set.

The world number 49 then broke 25th seed Safin twice in the second set.

Another break in the third was enough to wrap up a 6-3 6-3 6-3 win for Wawrinka.

The Swiss number two will face American Robby Ginepri, who destroyed Russian Teimuraz Gabashvili 6-2 6-3 6-1.

Eighth seed Tommy Robredo needed five sets to see off American Mardy Fish but eventually came through to earn a third-round meeting with Ernests Gulbis.

Fish, roared on the patriotic crowd, was 4-1 up in the fifth set before he capitulated.

"Credit to him, he stepped up his game, but I froze," admitted Fish.

"Against top players in the world, you just can't freeze like that. I think it had a lot to do with nerves."

Twelfth seed Ivan Ljubicic beat Andrei Pavel in straight set and now meets 20th seed Juan Ignacio Chela in the second round.

Veteran Spaniard Carlos Moya, playing his 12th US Open, was another player to reach the third round.

The 31-year-old 17th seed, who reached the last four back in 1998, saw off Russian Igor Kunitsyn in straight sets.