New York ? Roger Federer sneered, tossed his racket in disgust. Horror of horrors, he lost a set.

For most of the U.S. Open, the defending champion and top seed had seemed to sleepwalk through his matches, playing only as well as necessary, waking up and painting lines when pressed.

That was enough until he entered into a little trouble against Nicolas Kiefer on Tuesday. Under just the hint of pressure, Federer produced his best tennis of the tournament to beat Kiefer 6-4, 6-7 (3), 6-3, 6-4 and land safely in the quarterfinals.

Match point was a masterful final stroke – an inside-out forehand crosscourt that Federer tucked neatly in the corner, far from Kiefer’s reach.

“Federer’s play, for him, is like a B, B-plus, which for anyone else is an A-plus,” former champion John McEnroe said after leaving the broadcast booth.

Like an artist standing back to admire his work, Federer watched a replay on the giant screen of an acrobatic backhand pass that saved break point.

“I knew the moment I hit it that the ball is going to be in,” he said. “I knew that he’s not going to be there, and I knew that I’m back in the game, back in the match.

“I knew the importance of that shot. It was important from then to hold and not let it slip away. So I did well there.”

Lleyton Hewitt, the 2001 champion and runner-up to Federer last year, reached the quarters for the sixth straight year with a 6-1, 6-4, 6-2 victory over No. 15 Dominik Hrbaty.

Hewitt advanced to play Jarkko Nieminen, who became the first Finn to reach the quarters in a Grand Slam event with a 6-2, 7-6 (6), 6-3 victory over Spain’s Fernando Verdasco.

In the late night match in New York, Venus Williams fell to Kim Clijsters, 4-6, 7-5, 6-1.

“I wasn’t really thinking about the score,” said Clijsters, who trailed 4-6, 2-4 with Williams serving in the second set. “I just tried to keep fighting and just kept running for each ball.”

Women’s top seed Maria Sharapova also yielded a set for the first time in the tournament before beating fellow Russian Nadia Petrova, the ninth seed, 7-5, 4-6, 6-4, to reach the semifinals against Clijsters.

Against Petrova, each of the first two sets took almost as long, and the match ran 21â2 hours before Sharapova, shrieking on nearly every point, ended it with a break in the final game on a lunging backhand return that Petrova couldn’t handle.

“Wow! It’s absolutely amazing, I can’t believe I pulled this match out today,” Sharapova said.