Rafael Nadal suffered a "very painful" cramp on his right leg during a press conference at the U.S. Open in New York City on Sunday.

Nadal interrupted the conference unexpectedly, closing his eyes and grimacing and then falling to the ground, according to video of the event.

After 15 minutes of medical treatment, the Spanish tennis player resumed the news conference and assured the media that he is fit for the tournament and will continue training normally.

"It was just cramping in the right leg, in the front and in the back. It was very painful, that's all," Nadal explained. "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," he said.

Nadal had defeated one of his toughest rivals, David Nalbaldian, on Sunday before the news conference. On Tuesday he will play against Luxembourg's Gilles Muller in the fourth round.

Muscle cramps are sudden involuntary contractions or spasms in one or more muscles, according to the National Insitutes of Health. They often occur after exercise or at night, and can last a few seconds to several minutes. Stretching or gently massaging the muscle can relieve the pain.