US Open 2011: Rafael Nadal through as Nicolas Mahut quits

Nadal lost his only previous match against Mahut at Queen's in 2007

Defending champion Rafael Nadal reached the third round of the US Open when Nicolas Mahut retired at two sets down.

The Spaniard, 25, had lost his only previous encounter with Mahut, but led 6-2 6-2 when the Frenchman pulled out with an abdominal injury.

American Donald Young caused an upset as he beat Swiss 14th seed Stanislas Wawrinka in a fifth set tie-break.

And Andy Roddick saw off 18-year-old compatriot Jack Sock 6-3 6-3 6-4 in the night session on Arthur Ashe Stadium.

Earlier in the day, former champion and 18th seed Juan Martin del Potro defeated fellow Argentine Diego Junqueira 6-2 6-1 7-5.

"It's very tough to play against a friend," Del Potro said. "We live in the same town, we have known each other since we were five years old."

Nadal had to battle for a three-set first-round win over Andrey Golubev and was just thankful to get through his second round, even if it involved a retirement.

"I prefer to be in the third round," he said. "I spent a long time on court in the first round. For sure it's not nice to win like this, but that's sport sometimes."

He added: "I think I did a few things much better than in the first match. I tried to to play with my strategy, tried to have the control of the point with my forehand, tried to play a little bit more aggressive when I had the chance, and I think I did."

Young was the world junior number one in 2005 but has failed to build on that in the senior ranks, and he admitted the crowd had helped him to the best win of his career against Wawrinka.

"They were everything," said the 22-year-old. "I was kind of tired like midway through the third, fourth set.

"They were chanting my name, which is great, kind of like baseball chants. It just reminded me and made me feel great that all these people really wanted me to win here. It just pushed me through. I can't describe how great it was."

Spain's fifth seed David Ferrer eased past American James Blake 6-4 6-3 6-4, and 12th seed Gilles Simon of France beat Guillermo Garcia-Lopez 6-4 6-7 (4-7) 7-5 6-3.

Juergen Melzer, seeded 17th, was the another casualty as he went down to Russia's Igor Kunitsyn 3-6 6-3 1-6 6-2 7-6 (7-5).

John Isner won an all-American battle against Robby Ginepri 6-4 6-3 6-4.

In the men's doubles, British pairing Colin Fleming and Ross Hutchins enjoyed a superb win over Max Mirnyi of Belarus and Canadian Daniel Nestor, beating the second seeds 6-3 7-5.

"Great feeling to get the win this evening," said Fleming on Twitter. "Thanks for all the support & positive messages. Can't wait for 3rd round on Sunday!!!"

And compatriots Jamie Delgado and Jonathan Marray saw off Brazil's Marcelo Melo and Bruno Soares, the 12th seeds, 6-3 6-7 (8-10) 7-6 (7-2).