NEW YORK — 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 this year’s U. S. Open.

But in a match marked by wild swings of momentum, Clijsters proved the more dogged competitor and pulled off the stunning upset Sunday at Arthur Ashe Stadium, 6-0, 0-6, 6-4, to advance to the quarterfinals of the season’s final major.

Clijsters won the match with the defensive tenacity that marked her play during her prime, when she ascended to the world No. 1 ranking at age 20 in 2003, racing around the Ashe Stadium court to retrieve the blasts from one of the two hardest hitters in women’s tennis.

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 the other Williams sister — Serena, the tournament’s three-time and defending champion — in the semifinals.

Serena has yet to lose a set in her march toward Saturday’s prime-time women’s final. On Sunday she became the first to secure her quarterfinal spot, needing just 64 minutes to crush Daniela Hantuchova, 6-2, 6-0.

The victory was an occasion for Serena to don yet another statement T-shirt for her post-match news conference; this one, emblazoned with “Vicious! Ambitious! Delicious!”

For the 24th-ranked Hantuchova, it was yet another exercise in futility, her record against Serena now 1-8.

Hantuchova is hardly alone in being unable to match Serena’s raw power and iron will.

Since Clijsters and fellow Belgian Justine Henin retired (in 2007 and 2008, respectively), women’s tennis has lacked a worthy challenger for the Williams sisters, who have combined to win four of the last five major titles.

But Clijsters made a point of not reading too much into her victory Sunday.

“I know today that Venus wasn’t playing her best tennis,” Clijsters said. “In that sense, it’s hard to kind of compare anything in today’s match.”

Williams, the U. S. Open’s 2000 and 2001 champion, has been playing with her left leg heavily taped above and below the knee, apparently suffering from tendinitis. But she has refused to comment on the injury and, despite any limitations, is competing with Serena in doubles as well.

Though she lacked the consistency she once boasted, Clijsters looked for stretches as if she never left, firing back the best shots Williams could produce.

Clijsters, the 2005 U. S. Open champion, battled wrist injuries before retiring at 23, citing a desire to get married and start a family.

She did both and quickly found herself so busy that she didn’t give tennis much thought after the birth of her daughter, Jada.

But Clijsters said her curiosity was stirred earlier this year after being invited to participate in an exhibition at Wimbledon alongside Andre Agassi and Steffi Graf. And in the process of getting in sufficient shape to avoid embarrassing herself, Clijsters found herself testing the limits of her fitness as a new mom.

She played her first competitive match Aug. 10 in Cincinnati. With no world ranking to speak of, she was granted a wild-card entry for this year’s U. S. Open, where she hoisted the trophy four years ago.

In Sunday’s decisive third set, Clijsters got the early break. But Venus kept the pressure on, driving deep, sharply angled groundstrokes that yanked the Belgian all over the court.

Clijsters proved she hasn’t lost her knack for retrieving, making some spectacular defensive 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.’ ”

And she closed with a service winner out wide that Venus shoveled into the net.

Earlier, third-seeded Rafael Nadal advanced to the fourth round with a 7-5, 6-4, 6-4 victory over fellow Spaniard Nicolas Almagro.

But new concerns were raised about Nadal’s fitness after he halted play in the third set to get treatment for what apparently is a strained abdominal muscle.

Nadal apparently suffered the injury last month in Cincinnati. And Sunday he appeared to rein in the toss on his service motion and pare some speed from his first serves.

But he declined to discuss any aspect of his health afterward, saying he was weary of answering questions about injuries, alluding to the tendinitis that forced him to miss two months of the season, including Wimbledon, earlier this year.

“I am here to try my best every day,” Nadal said.

Also Sunday, sixth-seeded Juan Martin del Potro blasted 20 aces en route to his 6-1, 3-6, 6-3, 6-3 victory over Austria’s Daniel Koellerer.

Tournament organizers are trying to make a sad situation a little better for Sloane Stephens.

Stephens is the 16-year-old contender in the girls’ tournament whose father, former NFL running back John Stephens, was killed last week in a one-car accident in Louisiana.

It’s only over the past couple of years that Sloane has gotten to know her father, who was divorced from Sloane’s mother, Sybil Smith, when Sloane was young. John Stephens had been suffering from a degenerative bone disease before the accident.

After thinking about it for a while, Stephens decided to withdraw from the tournament to attend the funeral in Louisiana on Tuesday, but organizers stepped in and rescheduled her second-round match so she could attend, and still come back and play.

“I’m ready and emotionally prepared,” Stephens said. “He was sick. He wanted to get in contact. We had a good relationship. It was long-distance but he was my dad, I respected that and that was it.”

The Associated Press contributed to this report.