By Steve Ginsburg

NEW YORK (Reuters) - Defending champion Rafa Nadal strolled into the third round of the U.S. Open Friday when his opponent, Frenchman Nicolas Mahut, retired with an injury.

The Spaniard already had the match under control, leading 6-2 6-2, when Mahut quit in the first game of the third set, gifting the second seed a spot in the third round.

"He felt pain when he was serving," said Nadal. "That's what he told me. I wish him all the best. I hope to see him back on court very soon.

"It's not nice win like this, but that's sport sometimes."

In the third round Nadal will face Argentine David Nalbandian, who defeated Ivan Ljubicic of Croatia 6-4 1-6 6-3 6-2.

He was off the court in one-hour, 21 minutes.

"What's best? You never know," he said. "The best thing is to play very good in the next match and try to be in the fourth round. That's the only thing that's positive.

"The confidence is coming every day. I improved a little bit today. I have a day off tomorrow for practice, keep doing few things. I improved with the few days off.

"I did a few things better to prepare for this match. I have another day tomorrow. We'll see."

Against the 99th-ranked Mahut, who had beaten Nadal in their only other career meeting, the Spaniard had 22 winners and only five unforced errors.

Nadal and Nalbandian have split their four career matches, with Nadal winning their most recent encounter last year in Miami.

"He's dangerous," said Nadal. "He's a very good player. We know each other really good. We practice a lot together. He's a good friend. Will be a very good test for me."

(Editing by Julian Linden)