Nadal needed treatment on his blistered right foot during his third round match with Nalbandian [GALLO/GETTY]
Defending champion Rafael Nadal shocked the US Open on Sunday when he collapsed in an agony of cramps during a news conference.
His cramp came after his third round win over close friend David Nalbandian.
Andy Murray meanwhile stayed on course for a semi-final clash against the Spaniard.
Nadal, 25, was talking to journalists, two hours after his third round win, when he grimaced in pain, screwed up his eyes and slumped in the back of his chair, feeling his right leg.
The world number two, a winner of 10 Grand Slam titles, slipped to the floor while tournament medical staff were summoned.
Scores of reporters and camera crews were ushered out of the packed interview room deep inside the Arthur Ashe Stadium.
"It was just cramping in the right leg, in the front and in the back. It was very painful, that's all"
However, after 15 minutes of treatment, Nadal was back on his feet, insisting that the problem was merely cramping.
"It was just cramping in the right leg, in the front and in the back. It was very painful, that's all," said Nadal, who resumed his news conference standing instead of sitting.
A smiling Nadal said the incident will not affect his preparations for his fourth round match against Luxembourg's Gilles Muller, scheduled for Tuesday.
Muller, who reached the last 16 by beating Russia's Igor Kunitsyn 6-1, 6-4, 6-4, famously defeated Nadal at Wimbledon in 2005 before the Spaniard got his All England Club revenge this year.
British fourth seed Andy Murray, the 2008 runner-up, took his career record against Spanish left-hander Feliciano Lopez to six wins in six meetings thanks to an impressive 6-1, 6-4, 6-2 victory over the 25th seed.
Murray didn't concede a point on his own serve in a brutal first set on Sunday which set the tone for the one-sided tie.
Argentine 18th seed Juan Martin del Potro, the 2009 champion, who missed last year's title defence because of a serious wrist injury, saw his hopes dashed by Gilles Simon in a four-hour marathon.
The French 12th seed booked his place in the last 16 for the first time with a 4-6, 7-6 (7/5), 6-2, 7-6 (7/3) win over del Potro who committed 72 unforced errors and squandered three set points in the 10th game of the fourth set.
Another battle for Stosur
Samantha Stosur dropped an epic second-set tiebreaker but recovered to beat Maria Kirilenko 6-2, 6-7 (15/17), 6-3 on Sunday to reach the quarter-finals.
Russia's Kirilenko prevailed in what the WTA said was the longest women's singles tiebreaker at a Grand Slam tournament to knot the fourth-round match at a set apiece.
But it was the ninth seed from Australia who moved on to a quarter-final clash with second-seeded Russian Vera Zvonareva.
Zvonareva advanced with a 6-2, 6-3 victory over Sabine Lisicki, the big-serving German who had hoped to improve on her run to the Wimbledon semi-finals.
Instead Zvonareva, runner-up to Kim Clijsters here last year, needed just 77 minutes to wrap up her fourth career victory over Lisicki in as many meetings.
Stosur has won the last seven of her meetings with Zvonareva, but said she'd take nothing for granted.
"Now we're playing in a Grand Slam," Stosur said.
| Australian Sam Stosur has had a couple of tricky rounds already at the US Open [GALLO/GETTY]
"I'm sure she wants to get one back. Obviously it's a big moment for both of us in the quarters."
Stosur had already entered the record books this week when she toiled for three hours and 16 minutes to get past Nadia Petrova 7-6 (7/5), 6-7 (5/7), 7-5 in the third round - the longest women's singles match recorded at the US Open since the tiebreak era began in 1970.
Last year, Stosur saved four match points to win the latest-finishing women's match in US Open history in her fourth-round defeat of Russian Elena Dementieva.
Stosur said she didn't expect the long hours she'd been putting in to affect her against Zvonareva.
"I actually feel better after this match than what I did the last match," the 2010 French Open finalist said.
"Who knows how I'll feel tomorrow morning,but so far, so good."
Stosur, who had dominated in the opening set, said that once the tiebreaker was behind her she didn't find it too difficult to stay mentally strong.
"I knew if I was going to have a chance to win the match, I had to do that."