NEW YORK — A half-hour after getting past Venus Williams in a three-set tussle at the U.S. Open, Kim Clijsters had her hands full again.

Juggling an energy drink, a bottle of water and a snack, Clijsters was trying to keep an eye on her 18-month-old daughter, Jada, as the tyke scurried around the players' lounge.

Better keep the nanny on call: Mommy's got more work to do at Flushing Meadows.

Playing by far her biggest match since coming back after 2 1/2 years away from tennis, Clijsters knocked off the No. 3-seeded Williams, 6-0, 0-6, 6-4, yesterday in a match of wild momentum swings to reach the U.S. Open quarterfinals.

"It's still kind of hard to believe. But then again, I'm not trying to get carried away with it all," the 26-year-old Belgian said. "Just trying to focus on what I have to do, because the tournament's still going. I just want to keep focusing on my tennis."

And some tennis it is. Against Williams, a seven-time major champion, Clijsters displayed the same sort of booming groundstrokes and all-over-the-place court coverage that helped her win the 2005 U.S. Open and briefly reach No. 1 in the rankings before leaving the tour.

Only two mothers have won a Grand Slam singles title; the last was Evonne Goolagong Cawley at Wimbledon in 1980. Clijsters will be in the semifinals if she beats No. 18 Li Na of China.

"With the kind of training that she's put in, I knew this wasn't just for fun," said Clijsters' husband, Brian Lynch, an American who ended his professional basketball career in Belgium when she decided to unretire. "She was trying to make something happen here."

Consider that done, even if Williams appeared slightly hobbled at times by her heavily bandaged left knee, and her mother, Oracene Price, said afterward: "We all know she's just trying to go as far as she can. I don't know if she should have done that."

"I wasn't able to play 100 percent," Williams said.

Still, she went back out on court later yesterday, teaming with her younger sister Serena to win a third-round doubles match. The No. 2-seeded Serena is still defending her title in the singles tournament — hasn't dropped a set yet, even — after taking the last 10 games of a 6-2, 6-0 victory over No. 22 Daniela Hantuchova.

Serena Williams' bid for a 12th Grand Slam title will continue with a quarterfinal against No. 10 Flavia Pennetta of Italy, who saved six match points en route to eliminating No. 7 Vera Zvonareva, 3-6, 7-6 (6), 6-0, at night.

Williams-Clijsters was the main attraction yesterday, and the biggest piece of news elsewhere came when Rafael Nadal sought treatment from a trainer for a stomach muscle problem.

The third-seeded Nadal, trying to complete a career Grand Slam with a U.S. Open title, refused to talk about his health after beating No. 32 Nicolas Almagro, 7-5, 6-4, 6-4.

"I don't want to talk about injuries," Nadal said. "Sorry. No, no. I am a little bit tired to talk about injuries."

His next opponent, No. 13 Gael Monfils, advanced when Jose Acasuso quit because of left knee pain while trailing, 6-3, 6-4, 1-0.

No. 24 Juan Carlos Ferrero, the French Open champion and U.S. Open runner-up in 2003, moved on when his foe, No. 9 Gilles Simon, stopped playing because of a right knee injury, while winners included No. 2 Andy Murray, No. 6 Juan Martin del Potro, No. 7 Jo-Wilfried Tsonga, No. 11 Fernando Gonzalez and No. 16 Marin Cilic.

Murray overwhelmed 195th-ranked Taylor Dent of Newport Beach, Calif., 6-3, 6-2, 6-2, in yesterday's last match. The loss ended an inspiring run by Dent, who once was ranked as high as 21st, then was told by doctors he'd never play on tour again after a series of back operations.

Clijsters stepped away from the game in May 2007 after a series of injuries. She got married later that year, and gave birth to Jada in February 2008.

"I'm glad I made that choice," she said yesterday, "because a lot of beautiful things came out of it."

Clijsters' first official match was Aug. 10 — a win over 2007 Wimbledon runner-up Marion Bartoli — and because Clijsters only entered two tournaments before arriving in New York, she still isn't ranked by the WTA.

She needed a wild-card invitation to be able to play in the U.S. Open, and now is the first such woman to make the quarterfinals.

"She's come back fresh, rejuvenated and just ready to play and eager to play," observed Price, "and seeing the value of it more so than she did when she left."