Cramping Nadal, Roddick into fourth round, Del Potro out of US Open

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.

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."

"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).

Juan Martin Del Potro of Argentina, the 2009 US Open champion and the 18th seed this year, lost 4-6, 7-6 (5), 6-2, 7-6 (3) to Gilles Simon, the 12th seed from France, in a 3 hour, 57 minute match.

"It's very hard make a winner playing against Simon," said Del Potro.

"But I try until the end, and I play in lower level the tiebreaks. He take the chance and he close the match on fourth set," added the Argentinean, who lost both tiebreakers.

Donald Young became the second American player progressing in men's draw as the 22-year-old got into the tournament as a wild-card entrant, winning 7-5, 6-4, 6-3 against Juan Ignacio Chela, the 24th seed from Argentina, and next faces the winner between fourth-seeded Andy Murray of Britain and Feliciano Lopez, the 25th seed from Spain.