Recuperating former No. 1 Rafael Nadal appeared in danger, at times, of being brought to his troublesome knees by German journeyman Nicolas Kiefer on Friday night but trooped through a 6-0, 3-6, 6-3, 6-4 victory to reach the U.S. Open's third round for the fifth consecutive year.

Back on the Grand Slam circuit after being upset in the French Open and shelved by knee tendinitis for Wimbledon, Nadal breezed through the first set, but Kiefer began to display an uncanny anticipation, repeatedly catching Nadal out of position.

Ultimately, though, Kiefer was done in by his unreliable first serve (only 41 percent) and comparatively ragged play (60 unforced errors); Nadal had only 22 unforced errors as yet another night match went beyond midnight, ending at 12:15 a.m.

"Thank you very much for this atmosphere," Nadal told the Arthur Ashe Stadium crowd after the match. "I'm just very happy to be here and to be on this run."

Nadal, 23, healthy again but rusty, thus kept alive his pursuit of his first U.S. Open championship, the only Grand Slam tournament he has not won. His deepest run at Flushing Meadows came last year, when he was beaten by Andy Murray in the semifinals.

Nadal, who won three of the four major tournaments before the 2009 French, also ensured that all 16 of the top men's seeds would advance to the tournament's third round for the first time in any Grand Sl