US Open 2011: Rafael Nadal overcomes David Nalbandian test

World number two Nadal now faces Gilles Muller of Luxembourg

Defending US Open champion Rafael Nadal sealed his place in the fourth round by defeating Argentina's David Nalbandian in straight sets.

World number two Nadal beat Nalbandian 7-6 (7-5) 6-1 7-5 but was given a stern test in a 62-minute first set.

Nalbandian served for it at 5-4 but the Spaniard produced his best tennis to eventually take it into a tie-break, which he won.

Elsewhere, Gilles Simon knocked out 2009 champion Juan Martin Del Potro.

Simon, the 12th seed, fought back from a set behind to seal a 4-6 7-6 (7-5) 6-2 7-6 (7-3) triumph over the Argentine in a shade under four hours in the Louis Armstrong Stadium.

"I play at lower level in the tie-breaks," said Del Potro, who is still making his way back after a serious wrist injury. "He took the chance and he closed the match in the fourth set.

"Anyway, I'm glad to be here, playing this tournament. I feel a little sad now because I just lost. But I think my comeback is in good shape. I'll keep trying to improve my game."

He will play John Isner next after the American 28th seed beat compatriot Alex Bogomolov jnr 7-6 (11-9) 6-4 6-4.

In the opening match of the day on Arthur Ashe Stadium, Nalbandian broke Nadal in his third service game to take control of the first set, only to lose his nerve at the crucial time.

Serving for the set, Nalbandian double-faulted at break point, handing Nadal the lifeline his impressive tennis had deserved.

A tie-break was required and it was Nadal who came out on top, thanks in part to another Nalbandian double fault at 3-3, to take the early advantage on a hot afternoon in New York.

Nadal turned on the style and the power in the second set, breaking his opponent three times to take a two-set lead, and he then recoverd from dropping serve in the opening game of the third.

The 10-time Grand Slam champion was unexpectedly broken again when serving for the match, but with Nalbandian needing to hold serve to take the set into a tie-break, the Argentine double faulted to hand Nadal the match.

"It was a tough day," said Nadal. "David is a fantastic player and a close friend who has had a tough time with injuries.

"The first set was key when I came back when he was serving for the set. I am happy because I played my best match of the tournament so far."

Nadal, who collapsed with cramp in his post-match press conference before making a full recovery, will now take on Gilles Muller of Luxembourg, who beat Russia's Igor Kunitsyn 6-1 6-4 6-4.

Fifth seed David Ferrer of Spain needed less than two hours to beat Florian Mayer of Germany 6-1 6-2 7-6 (7-2) and he will next play 2003 champion Andy Roddick, who overcame French wildcard Julien Benneteau 6-1 6-4 7-6 (7-5).

There was more good news for the host nation as unseeded American Donald Young beat Juan Ignacio Chela of Argentina 7-5 6-4 6-3.