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 Sunday 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 Na Li 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 Sunday, teaming with her younger sister Serena to win a third-round doubles match.
Serena is still waiting for her first true test of this U.S. Open.
Serena, who is seeded second at the Open, has reached the quarterfinals at 11 of the past 12 major tournaments and won the title at three of the past four.
"I just want to keep this level and just stay focused," Serena said.
With the score 2-all against Hantuchova, Williams hit three aces to hold for a 3-2 lead -- and she wouldn't lose a game the rest of the way.
She has won all eight sets she's played this year at Flushing Meadows.
"I tried to relax," Serena said after improving to 8-1 against Hantuchova over their careers. "Sometimes I'm such a perfectionist, I put too much stress on myself. I was like, 'Serena, relax!'"
She finished with eight aces and a 27-9 advantage in winners in Sunday's first match in Arthur Ashe Stadium.
Serena's 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.
No. 1 Dinara Safina was upset in the third round Saturday night, meaning Serena is the highest-ranked woman left in the field. Safina will remain atop the rankings, though, even if the younger Williams were to win a second consecutive U.S. Open title and fourth overall.
"I didn't even see that result until this morning, so I haven't thought much about it," Williams said. "I'm not focused on who wins and who loses. I'm just focused on hopefully what I can do."
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 Sunday, "because a lot of beautiful things came out of it."
To hear Clijsters tell it, she never gave a shred of thought to ending her retirement until being asked to participate in exhibition matches under the new roof on Wimbledon's Centre Court in May. Eager to acquit herself well, she began working out and practicing -- and the desire to compete for real came surging back.
Her first official match came 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."
Clijsters credits her time away with improving her mental strength on the court, and it came in handy on a cloudy, windy afternoon.
Venus got off to an inauspicious start, putting in only 3 of 12 first serves in the opening game, in which she double-faulted twice and sprayed shots wildly. Clijsters took the first six games, Williams took the next seven, then Clijsters the next three.
"Very weird, right?" is how Clijsters described those ebbs and flows.
Indeed, there hadn't been a 6-0, 0-6 start to a women's match at the U.S. Open since 1975.
Clijsters' forehand went away in the second set, then returned with great effectiveness in the third. Consecutive winners off that wing helped her gain the only service break of the final set, one that put her up 2-1 when Williams double-faulted.
With the crowd pulling for the Belgian more than the American, Clijsters erased two break points while serving for the match, then ended it with a 101 mph service winner.
Yes, Mom's still got it.
"Tennis is a great sport, but I'm just happy that we have a family and I can balance both," Clijsters said in an on-court interview, drawing a roar from fans.
After noting that Jada isn't concerned with her wins or losses, Clijsters elicited more yells of support when asked about changes she noticed when returning to the sport.
"I only just started watching tennis at the start of this year, to be honest," she replied. "I didn't really have that much time with a baby running around, and I was happy just to sleep when she was sleeping."
What parent can't relate to that?
The Associated Press contributed to this story.