NEW YORK (AP) – 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. He made up shots as he went along – a sprinkling of aces at various speeds and angles, a backhand pass that got him out of trouble, a volley that came out of nowhere between yawns.

That was enough until he got into a little trouble against Nicolas Kiefer on Tuesday. Suddenly Federer had a reason to elevate his game and stir some emotion. Now he swept in toward the net, pounded winners from the baseline, stopped wasting time and effort. 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.

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 the winner of the Venus Williams-Kim Clijsters match.

Sharapova won her first four matches in an average of 59 minutes. Against Petrova, each of the first two sets took almost as long, and the match ran 21/2 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,” said Sharapova, who lost in the third round last year and the second round in her first U.S. Open two years ago. “So many ups and downs. … I just found a way to fight. A lot of credit to Nadia, she played an amazing match.

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 win over No. 15 Dominik Hrbaty, who drew more attention for his pink peekaboo shirt than his play.

“It made it a lot easier for me to beat him today,” Hewitt said. “I just couldn’t lose to a bloke wearing a shirt like that.”

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. Federer next plays 11th-seeded David Nalbandian of Argentina.

, a 4-6, 7-6 (4), 6-4, 6-2 victor over Italian Davide Sanguinetti. Federer said before that match he would prefer Nalbandian, “because I have a bad record against him.”

Former champion John McEnroe liked that.

“Most guys would like to play the guy they have the winning record against,” McEnroe said when he stepped outside the broadcast booth. “That’s what you love about Roger – he embraces the challenge.”

Nalbandian holds a 5-2 record against Federer, though Federer won the last two meetings in 2003 and 04.

“I like to play against guys who have beaten me, especially early in the career, try to get them back,” Federer said. “He was a tough opponent for me for a long, long time.”

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