By Larry Fine

NEW YORK (Reuters) - Defending champion Rafa Nadal outbattled David Nalbandian of Argentina on Sunday to reach the fourth round of the U.S. Open before sending a scare into the National Tennis Center when he collapsed from cramping.

Nadal defeated his friend and practice partner 7-6 6-1 7-5 but two hours later he was overcome by pain during a post-match news conference and slid from his chair to the floor behind the desk as officials summoned medical personnel.

Journalists were ordered to leave the room and the lights were turned down, leaving everyone in doubt as to Nadal's condition, although live footage of the room was shown on closed circuit television.

Nadal could be seen getting his leg massaged, while another man gave the Spaniard ice and fluids before they helped him to his feet.

He returned to the conference room shortly after, smiling, to explain what happened.

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

The post-match episode provided much more suspense than what transpired on-court at Arthur Ashe Stadium.

Second-seeded Nadal breezed through the middle set, though he was made to fight hard on both ends of the match on a hot, humid day at Flushing Meadows.

"I was happy about almost everything today," Nadal told reporters before his collapse.

"I think my movements worked pretty well, and the forehand worked really well, and the backhand, too."

Nadal, who has yet to lose a set but has been far from his sharpest, said he was encouraged by his progress.

"I think I played every day better," he said. "Today, for moments I played, in my opinion, very, very positive level, very high level."

The Spaniard trailed 5-4 with Nalbandian serving for the opening set when he broke the Argentine's serve and forced a tie-break, which he won 7-5.

After sailing through the second set, Nadal took a medical time out during the changeover at 1-1 in the third set to have his right foot taped for a blister.

Nadal fell back in step and took a 5-2 lead, but Nalbandian roared back to make it 5-5. The Spaniard held and then broke serve on his third match point when Nalbandian double faulted.

"I think I played my best match of the tournament, so I'm happy for that," said Nadal. "Every day is tougher and tougher."

Next up for Nadal will be Gilles Muller of Luxembourg, a straight sets winner against Russian Igor Kunitsyn.

(Editing by Julian Linden)