FLUSHING MEADOWS, N.Y. - Eighteen months after giving birth to her first child and less than one month after coming out of retirement, Kim Clijsters was an unlikely candidate for toppling Venus Williams at the U.S. Open.
But in a match marked by wild momentum swings, Clijsters proved the more dogged competitor Sunday at Arthur Ashe Stadium, winning 6-0, 0-6, 6-4 to reach the quarterfinals.
Clijsters won with the defensive tenacity that marked her play during her prime, when she ascended to the No. 1 spot at age 20 in 2003.
Clijsters' reward is a quarterfinal meeting with China's Li Na, a 6-2, 6-3 victor over Italy's Francesca Schiavone. Should Clijsters defeat Li, she'd likely face Venus' sister - Serena, the event's three-time champion - in the semis.
Serena has yet to lose a set. On Sunday, she crushed Daniela Hantuchova 6-2, 6-0 to secure a quarterfinal against No. 10 Flavia Pennetta. At her postmatch news conference, Serena donned a statement T-shirt: "Vicious! Ambitious! Delicious!"
But Clijsters made a point of not reading too much into her victory. "I know today that Venus wasn't playing her best tennis," Clijsters said.
Venus, the U.S. Open's 2000 and 2001 champion, has been playing with her left leg taped above and below the knee, apparently suffering from tendinitis.
In the third set, Clijsters got the early break. But Venus kept driving deep, sharply angled groundstrokes that yanked the Belgian all over the court. Clijsters proved she hasn't lost her knack for retrieving, making spectacular plays. But she said she was trembling after falling behind 15-40 while serving for the match.
"My arm felt like it was 50 pounds," Clijsters said. "I just told myself, 'Don't give it away like that.'"
So she closed with a service winner out wide that Venus shoveled into the net.
Earlier, men's third seed Rafael Nadal advanced to the fourth round with a 7-5, 6-4, 6-4 victory over Nicolas Almagro.
But new concerns were raised about Nadal's fitness after he halted play in the third set to get treatment for what appeared to be a strained abdominal muscle. He appeared to rein in the toss on his service motion and pare some speed from his first serve.
He declined to discuss his health afterward. "I am here to try my best every day," he said.
Late Saturday, women's No. 1 Dinara Safina was upset in the third round by 72nd-ranked Petra Kvitova 6-4, 2-6, 7-6 (7-5) when the match was moved from the main stadium because of the late-running afternoon session. Ahead 6-5 in the third set, Safina had three match points but lost them all. She committed 39 unforced errors and nine double faults.
On the men's side, No. 14 Tommy Robredo ousted Tampa resident and 21st-ranked James Blake 7-6 (7-2), 6-4, 6-4. Blake, who had 45 unforced errors, injured an ankle in the first set. "But that isn't an excuse," he told the New York Daily News. "That's part of sports."
The Associated Press contributed to this report.
Men: Rafael Nadal (3) d. Nicolas Almagro (32) 7-5, 6-4, 6-4; Juan Carlos Ferrero (24) d. Gilles Simon (9) 1-6, 6-4, 7-6 (7-5), 1-0, retired
Women: Serena Williams (2) d. Daniela Hantuchova (22) 6-0, 6-2; Kim Clijsters d. Venus Williams (3) 6-0, 0-6, 6-4
Today's key matches
Men: Roger Federer (1) vs. Tommy Robredo (14); Novak Djokovic (4) vs. Radek Stepanek (15)
Women: Svetlana Kuznetsova (6) vs. Caroline Wozniacki (9), Nadia Petrova (13) vs. Melanie Oudin
TV: 11 a.m., Ch. 10; 7 p.m., ESPN2