Moving well, stinging shots with both forehand and backhand and matching every bit of Williams' power, the Belgian star offered a startling referendum on the state of her game in her first Grand Slam appearance since having a baby 18 months ago.
It may have also said something about the true state of Williams' left knee, which she hurt in the opening round, but had refused throughout the tournament to use as an excuse.
"It was unbelievable. I don't know what to say," Clijsters said. "It was such a weird match, especially those first two sets. But after I lost the second at 6-0, I said, let's start over and start a new match."
Indeed, the match began the way many Sundays do in Queens - with a couple bagels. It took a grand total of 50 minutes to complete those first two sets, but both players regrouped from that bit of awkwardness and played some of the most compelling, solid tennis of the tournament so far.
Clijsters grabbed an early break for a 3-1 lead in the third, helped by one of Williams' five double-faults to close it out. Clijsters served out the match from there, though it was anything but routine.
She fell behind 0-30 on her serve at 5-4, but just kept banging away. She got it to 30-40, then hit a shot deep into the corner that Williams couldn't handle. She forced an error at deuce with another deep groundstroke, then skidded a service winner off the line on the backhand side for the win.
Her reaction was one not so much of surprise, as a smile that seemed to say "I told you so." She became the first female wild-card entrant to reach the U.S. Open quarterfinals. One promising slice of history: This marked the third meeting between Clijsters and Williams at the U.S. Open, and each previous time, the winner has gone on to take the championship.
"I've been working really hard the last seven, eight months and I'm enjoying it," Clijsters said. "It's something that's really important for myself, as long as I can focus on tennis and have fun on the outside as well."
Clijsters retired in 2007 to start a family and hadn't seen Grand Slam action since that year, but is quickly re-establishing herself as one of the few who can move well enough and hit hard enough to hang with the Williams sisters.
Mother of 18-month-old Jada, Clijsters is trying to join Margaret Court and Evonne Goolagong as the third mother to win a Grand Slam singles title.
Who said motherhood and topflight tennis can't mix? "We have help, so that's great," Clijsters said. "It's fun. To her, it doesn't matter whether I win or lose. She's just happy to see me and that's great."
Clijsters came into the U.S. Open without enough tournaments under her belt to receive a ranking, and now finds herself two wins away from becoming the first unseeded player to reach the finals of the Open since Williams in 1997.
"I was really glad coming back that a few of the girls who were there when I was playing well are still on top right now," Clijsters said. "And it's great to see some of the newcomers doing well, too. It's fun for me to be part of the change the sport is going through."
On the men's side, No. 3 Rafael Nadal overcame a 10-minute medical break for an injury to his stomach muscles to defeat 32nd-seeded Nicolas Almagro, 7-5, 6-4, 6-4.
Nadal missed Wimbledon with knee injuries, and now must deal with injured abs that first cropped up last month in Cincinnati.
"I don't want to talk about injuries," Nadal said. "Sorry. No, no, I am a little bit tired to talk about injuries. I am here to try my best every day."
In other men's matches, No. 11 Fernando Gonzalez advanced, as did No. 6 Juan Martin del Potro and No. 7 Jo-Wilfried Tsonga. No. 13 seed Gael Monfils moved on when Jose Acasuso retired with an injury - not immediately disclosed - in the third set, while 24th-seeded Juan Carlos Ferrero won when ninth-seeded Gilles Simon retired in the fourth set with a right knee injury.
The biggest buzz on this blustery day in Flushing Meadows belonged to Clijsters, the Belgian, who was clearly the fan favorite at Ashe Stadium - an honor usually reserved for an American going against a foreigner at this tournament.
The Clijsters win injected another twist into a tournament that is not going to form. No. 2 Serena Williams is the only top-5 seed remaining. Meanwhile, 70th-ranked Melanie Oudin, the 17-year-old from Marietta, Ga., is turning into a star.
Clijsters shook up the side of the bracket that practically had Williams-vs.-Williams penned in for the semifinals. Her next match is against 18th-seeded Li Na of China, with a possible meeting with Serena after that.
Before her sister lost, Serena Williams cruised through her fourth-round match, winning the final 10 games in a 6-2, 6-0 rout over No. 22 Daniela Hantuchova.
"I just want to keep this level and just stay focused," Williams said.
Shortly after Venus Williams lost, she joined her for a doubles match - a sign that her ailing knee can't be that bad. Debatable, said her mother, Oracene Price.
"We all know she's just trying to go as far as she can," Price said. "I don't know if she should have done that."