-- Just as the women's half of this season's U.S. Open was starting to lull viewers to sleep, with one favorite after another rolling to victories devoid of drama, Kim Clijsters roared back after coming within two games of defeat to topple defending Wimbledon champion Venus Williams in a three-set battle that stretched well past midnight.

Clijsters's 4-6, 7-5, 6-1 victory, which sends her into the semifinals of a Grand Slam event for the first time this season, validates her hard-fought comeback after wrist surgery nearly ended her career last season.

The 2-hour 5-minute match was riddled with errors (51 by Williams; 38 by Clijsters) and 13 service breaks. But it offered a stirring finale to an evening session in which top-seeded Maria Sharapova turned back her first real challenge of the tournament with a 7-5, 4-6, 6-4 victory over fellow Russian Nadia Petrova to advance to the tournament's semifinals for the first time in her young career.

The ninth-seeded Petrova made it interesting, pouncing on Sharapova's lapse in the second set to become the first woman to win a set against the 2004 Wimbledon champion at this tournament. But with a chance to pull even in the third set, serving at 4-5, Petrova smacked a series of errant shots to hand Sharapova the victory.

"I can't believe I pulled this match out today," Sharapova chirped to the crowd at Arthur Ashe Stadium between giggles and deep breaths following the 2-hour 30-minute match. "A lot of credit to Nadia; she played amazing. I don't know what I'm doing here."

The 18-year-old Sharapova will face the fourth-seeded Clijsters in one semifinal; the other pairing will be determined Wednesday, when Frenchwomen Amelie Mauresmo and Mary Pierce face off, and top-ranked Lindsay Davenport plays last year's finalist, Elena Dementieva.

Clijsters beamed when Williams's final forehand sailed long. It had been a hard slog to claw back into the match after Williams bolted to a 6-4, 4-2 lead.

"I wasn't really thinking about the score, I guess, and tried to keep fighting, keep running for each ball," Clijsters told TV viewers and the few thousand fans who stayed to the end. Widely viewed as the best women's player to never have won a Grand Slam title, Clijsters prevailed in the wee hours of the night by being the fitter woman, winning 11 of the last 13 games.

The result means that for the third consecutive year the U.S. Open will be without a Williams sister for its final weekend.

The turning point came in the second set. With Williams serving at 4-2, the match slowly devolved into a tortured battle of errors and nerves. Forehands ran amok; serves collapsed. There were seven breaks of serve. And Clijsters served for the set twice, at 5-4 and 6-5, finally clinching it when Williams netted a forehand.

"I feel like I was playing decent, and she started playing really bad, and it totally threw me off," Williams said. "She started hitting short balls and weird stuff. It threw me off, and the next thing I knew I was playing as bad as she was."

Sharapova's match featured even weirder stuff, as a stray tennis ball dribbled onto center court, apparently tossed by a fan, during the middle of a rally. Sharapova stopped playing, expecting the umpire to order a replay of the point. But the official didn't see the ball and awarded the point to Petrova. Sharapova shrieked in disbelief, but her appeals for a replay went nowhere. She won the next point, but lost the set.

The controversy seemed to rile her, and she rolled to a 5-2 lead in the deciding set. Petrova channeled her anger into her forehand, yanking the long-legged Sharapova from one corner of the baseline to the other. After pulling to 4-5, Petrova's nerves proved her undoing (she committed 50 errors to 22 winners).

"We had break points, we had really bad points. We had a lot of unforced errors. It was a very tough match mentally," said Petrova.

The top seed in the men's draw, Roger Federer, also faced his first real challenge, dropping a set to Nicolas Kiefer before prevailing, 6-4, 6-7 (7-3), 6-3, 6-4, to move into the quarterfinals, where David Nalbandian of Argentina awaits.

Federer has been the one constant in men's tennis these last years, as inevitable a presence in the late stages of a major tournament as chair umpires and sweat. And though he has barely drawn comment through the early rounds of the U.S. Open, he lurks behind every story that unfolds on these courts, cruising toward Sunday's final with the stealthy purpose of a shark.

"Nine times out of 10, I think, if you're going to win the tournament, you're going to have to come up against Federer at some stage at the moment," said third-seeded Lleyton Hewitt, who remains on track to meet Federer in Saturday's semifinals. "It's just a matter of getting through your matches."

Kiefer has given Federer trouble before. He was the only player to snag a set from the Swiss during his waltz to a third consecutive Wimbledon title this summer. But neither that achievement, nor the 3-hour 2-minute challenge Kiefer gave him Tuesday, offered any solace.

"I mean, we are on Earth," Kiefer said, "but he's playing on a different planet."

Kim Clijsters, above, defeats Venus Williams, 4-6, 7-5, 6-1, to advance to a semifinal against Maria Sharapova.