Rafael Nadal of Spain lunges for a return to David Nalbandian of Argentina during their match at the US Open tennis tournament in New York, Sept 4, 2011. [Photo/Agencies]

NEW YORK - Rafa Nadal sent a scare through the US Open on Sunday when he collapsed during his post-match news conference because of severe cramping in his leg.

The defending champion was speaking to reporters after his third round win over David Nalbandian when he suddenly stopped talking, then covered his face with his hands and slumped to the floor.

Journalists were ordered to leave the room and the lights were turned down as medical staff were called in to treat the Spaniard, who had slowly slid from his chair and on to the floor behind a desk.

A trainer massaged his right leg while another man gave Nadal ice and fluids. Nadal remained on the floor for about 10 minutes before he was eventually helped to his feet.

He returned to the conference room shortly after to explain what happened, smiling and playing down the incident.

"I just have cramping in my leg. That's all," he said. "I just have cramping in front and behind. That's why. It was so painful. That's all."

Nadal had just beaten Nalbandian 7-6 6-1 7-5 on a baking hot day at Flushing Meadows to book his place in the fourth round and was not the only player struggling with the heat.

Italy's Flavia Pennetta was so overcome by the humidity on court that she began dry retching when she was serving for the match in the second set against China's Peng Shuai.

Pennetta, who had upset Maria Sharapova in the third round, looked to be in danger of losing her way when she failed to hold her serve then fell behind 6-2 in the tiebreaker.

But the 29-year-old Italian quickly regained her composure and reeled off the next six points in a row to win 6-4 7-6 and advance to a quarter-final with unseeded German Angelique Kerber.

"I was feeling really bad," said Pennetta. "I think was because it's really humid today. It's hot. And also, when you are there you have a lot of emotion in the court.

"My body just need to breathe, and I starting maybe to have the sensation to throw up. But doesn't, without nothing inside, so it didn't come out."

Rafael Nadal of Spain celebrates his win over David Nalbandian of Argentina during their match at the US Open tennis tournament in New York, Sept 4, 2011. [Photo/Agencies]

There was no sign of what was to come from Nadal as he eased his way past Nalbandian in straight sets.

By his own lofty standards, it was not a great performance by Nadal but he survived against an experienced and difficult opponent and has yet to drop a set in the tournament.

His title defence remains on track although the second seed needs to keep improving. His serve, which provided the foundations for his win last year, remains scratchy and he is still struggling to regain his confidence after losing the Wimbledon final to Novak Djokovic.

"I think I have played better every day," Nadal said.

"Today, for moments, I played, in my opinion, at a very, very positive level, very high level."

Nadal's next opponent is Gilles Muller of Luxembourg, who beat Russia's Igor Kunitsyn 6-1 6-4 6-4.

Another Spaniard, fifth seed David Ferrer, also advanced to the last 16, defeating Florian Mayer of Germany 6-1 6-2 7-6. Ferrer will now play former US Open champion Andy Roddick, a 6-1 6-4 7-6 winner over Julien Benneteau of France.