NEW YORK, Sept. 4 -- Defending champion Rafael Nadal and former winner Andy Roddick both marched into the fourth round of US Open with straight-set victories.

Nadal, the second seed, won 7-6 (5), 6-1, 7-5 in 2 hours, 39 minutes against David Nalbandian of Argentina.

About two hours later, the 25-year-old Spaniard suddenly slumped to the floor under a large table because of right leg cramps during a press conference.

"I just have cramping in my leg, that's all," Nadal said in a statement after the interview room was locked for about 10 minutes while trainers and his family attended to him. "That's why I fell. It was so painful."

Rafael Nadal of Spain hits balls in the crowd after after his opponent Nicolas Mahut of France retired with an injury at the U.S. Open tennis tournament in New York, September 2, 2011.

"It could have happened anywhere, it was just bad luck it happened here," Nadal said in Spanish while standing in front of the interview podium.

Nadal will play Gilles Muller of Luxembourg for a place in the quarterfinals at the final Grand Slam tennis tournament of the year. Muller breezed past Igor Kunitsyn of Russia 6-1, 6-4, 6-4.

Roddick, the 21st seed from the United States and the champion in 2003, won 6-1, 6-4, 7-6 (5) against Frenchman Julien Benneteau and set up a match against David Ferrer, the fifth seed from Spain for a quarterfinal place.

Ferrer moved on also in straight sets against 26th seed Florian Mayer from Germany 6-1, 6-2, 7-6 (2).

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