Defending champion Rafael Nadal strolled into the third round of the US Open when his opponent, Frenchman Nicolas Mahut, retired hurt.

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 place in the next round.

Nadal, who won in New York last year to complete his set of Grand Slam titles, next faces either David Nalbandian or Ivan Ljubicic.

Juan Martin Del Potro defeated Argentine compatriot Diego Junqueira in straight sets to reach the third round at Flushing Meadows.

The 18th seed, who won the tournament in 2009 but has been beset by injuries almost ever since, triumphed 6-2 6-1 7-5 in a sunny New York.

The 30-year-old Junqueira is ranked 104th in the world and simply did not possess the weapons to hurt his opponent, who like him was born in Tandil.

Del Potro was dominant in the opening two sets, serving harder, hitting a greater number of winners and attacking the net frequently.

However he made heavier weather of the third set, with Junqueira breaking him when serving for it then almost repeating the feat before Del Potro saw the match out.

Del Potro was more imposing than his rival, but his tailing off late on gives rise to concerns over whether he could physically handle a run to the final of this season's tournament.

Del Potro might be forgiven for thinking he is the forgotten man of the tournament - while the towering Argentine was hailed as the next big thing in men's tennis after he won the title two years ago, beating Roger Federer in a classic five-set final, he has been an almost anonymous figure this time.

While Federer, Rafael Nadal and Andy Roddick, the only other players in the men's draw to have won the men's titles, have played all their matches in the Arthur Ashe Stadium, Del Potro has not played on the centre court for two years.

He was unable to defend his title last year because of a nine-month-long injury - but his first two matches have been played on smaller courts during the day this year.

However he said he had no complaints about being overlooked for the feature night matches and said the organisers were right to save the red carpet treatment for the bigger names.

"I've never played at night here. It would be nice, but I should win more matches to do that," he said.

"They have Federer, (Novak) Djokovic. I'm in the second part of the draw, so I play same day like Nadal, (Andy) Murray, Roddick, and they are fantastic players to play at night.

"So I think they are doing a good schedule."

Del Potro will certainly get his chance on the big stage if he continues in the tournament.

"I played better than my first round [against Junqueira]," he said.

"I was more confident with my forehand especially in the beginning of the match, and it is always important to go through.

"It doesn't matter if you play good or not in the first round, as long as you win. Now I'm in the third one, so I need to improve."