Then slowly - and scarily to those watching, because it was unclear at that moment to anyone but Nadal himself exactly what was wrong - the defending champion slithered out of the chair and went down to the ground, hidden from view by a table. Within minutes, Nadal was sitting up, and then standing, after being given bags of ice to soothe his painful leg and bottles of water and Gatorade to drink.
Even if it all amounted to nothing serious from a medical standpoint - as Nadal and his manager would later insist, chuckling - it was a bizarre scene, one at least as memorable as anything that took place on court Sunday at the year's last Grand Slam tournament.
'It's bad luck it happened here,' Nadal said, 'and not in the locker room.'
His point was that tennis players often deal with cramps, particularly after competing in the sort of conditions Nadal did while beating 2002 Wimbledon runner-up David Nalbandian 7-6 (5), 6-1, 7-5 on a muggy afternoon with the temperature in the 80s.
Nadal next faces 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.
'It's just something that happens. It's just unfortunate it happened in front of you all,' 2003 US Open champion Andy Roddick told reporters after a straight-set victory over Julien Benneteau to get to the fourth round. 'Every single player in there has had that happen before. Every single one. What we do - we run around, run miles and miles and miles and miles on the tennis court in nasty weather - (and) you throw nerves in there. I mean, it happens. As long as it doesn't happen during a match, you're fine.'
With No. 28 John Isner and unseeded Donald Young also winning Sunday, and No. 8 Mardy Fish advancing Saturday, Roddick is part of the first quartet of American men to reach the US Open's fourth round since 2003.
Others moving on included 2008 runner-up Andy Murray, who beat No. 25 Feliciano Lopez 6-1, 6-4, 6-2 at night; No. 5 David Ferrer, Roddick's next opponent; No. 12 Gilles Simon, who got past 2009 champion Juan Martin del Potro 4-6, 7-6 (5), 6-2, 7-6 (3); and unseeded Gilles Muller, who will face Nadal for a quarterfinal berth.
Murray now faces American wildcard Donald Young, the former world junior number one, who reached the fourth round at a Grand Slam for the first time with a 7-5, 6-4, 6-3 win over Argentine 24th seed Juan Ignacio Chela.
Spanish fifth seed David Ferrer, a semi-finalist in 2007, beat German 26th seed Florian Mayer 6-1, 6-2, 7-6 (7/2) and next tackles 2003 winner Andy Roddick.
The American 21st seed breezed past French wildcard Julien Benneteau 6-1, 6-4, 7-6 (7/5) and then laughed off the incident involving Nadal.
Nadal wasn't the only player who appeared to be bothered by Sunday's heat and humidity: No. 26 Flavia Pennetta of Italy felt as though she needed to throw up right out there on court during her 6-4, 7-6 (6) victory over No. 13 Peng Shuai of China.
'It was a bunch of things: the heat, the tension,' said Pennetta, who knocked off three-time major champion Maria Sharapova in the third round. 'It's not normal, but it happens.'
Pennetta now plays Angelique Kerber of Germany, who defeated Monica Niculescu of Romania 6-4, 6-3.
When a reporter began a question by asking Pennetta about beating Kerber when they played on clay at Bastad, Sweden, less than two months ago, Pennetta cut him off.
'This year? Really? I didn't know that,' Pennetta said. 'I thought I'd never played her. I swear to God.'
Fifth-seeded Sam Stosur, the 2010 French Open runner-up, isn't likely to forget her 6-2, 6-7 (15), 6-3 victory Sunday night over No. 25 Maria Kirilenko - or at least the second-set tiebreaker. The 17-15 score made it the longest tiebreaker played by two women at any Grand Slam tournament, according to the WTA.
Kirilenko won that set - saving five match points in the process - to force a third, and TV broadcaster John McEnroe declared of the tiebreaker: 'There, in a nutshell, is why this is such a great sport.'
In the quarterfinals, Stosur will play 2010 US Open and Wimbledon finalist Vera Zvonareva, who defeated No. 22 Sabine Lisicki 6-2, 6-3 in the last match of the night at Arthur Ashe Stadium.
Much earlier, on that same court, Nadal dealt with two too-close-for-comfort sets and was treated by a trainer for a blister on his right foot during an injury timeout.
The 76th-ranked Nalbandian went up a break in the fifth game and served for the first set at 5-4, but he double-faulted on break point. Then, at 3-all in the tiebreaker, Nalbandian double-faulted again, helping the second-seeded Nadal nose ahead. After a quick second set, Nalbandian broke to begin the third, but Nadal broke right back and wound up finishing things in a little more than 2 1/2 hours.