Defending champion Rafael Nadal collapsed with cramp during a news conference after his third-round win over Argentina's David Nalbandian.
The Spaniard, 25, had won the match 7-6 (7-5) 6-1 7-5 in hot afternoon conditions on Arthur Ashe Stadium.
He was talking to the Spanish media after the match when he slumped back in his chair and slid to the floor.
Medical staff were summoned and after 15 minutes of treatment he resumed the news conference.
"It was just cramping in the right leg, in the front and in the back. It was very painful, that's all," said Nadal.
"I will train normally on Monday. It was just a normal cramp that could have happened anywhere, but it happened in the press room. Anywhere else, nobody would have noticed."
Nadal will play Luxembourg's Gilles Muller in the fourth round at Flushing Meadows on Tuesday.
And he posted a video message on Facebook on Sunday evening, saying: "I had some cramp in the press conference but I am fine, I'm in the hotel having some rest and recovering."