Moving well and matching every bit of Williams' power, the former top-ranked Clijsters provided a startling update on the state of her game.
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," said Clijsters, the 2005 U.S. Open champion. "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 of bagels. It took 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, helped by one of Williams' five double-faults to close it out, then served out the match, though it was anything but routine.
The Belgian 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 101 mph (163 kph) 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."
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."
Mother of an 18-month-old daughter, Jada, Clijsters is trying to join Australians Margaret Court and Evonne Goolagong Cawley as the third mother to win a Grand Slam singles title.
"We have help, so that's great," Clijsters said of her motherly duties. "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."
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 American 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 Williams 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.
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.
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."
His next opponent, No. 13 Gael Monfils of France, who advanced when Argentina's Jose Acasuso quit because of left knee pain while trailing 6-3, 6-4, 1-0.
No. 24 Juan Carlos Ferrero of Spain, the French Open champion and U.S. Open runner-up in 2003, moved on when his opponent, No. 9 Gilles Simon of France, stopped playing because of a right knee injury, while winners included No. 2 Andy Murray of Britain, Argentina's No. 6 Juan Martin del Potro, No. 7 Jo-Wilfried Tsonga of France, No. 11 Fernando Gonzalez of Chile and 16th-seeded Marin Cilic of Croatia .
Murray overwhelmed 195th-ranked Taylor Dent of the United States 6-3, 6-2, 6-2 in Sunday'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 in February 2008.
"I'm glad I made that choice," she said Sunday, "because a lot of beautiful things came out of it."
Her 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.
Clijsters credits her time away with improving her mental strength on the court, and it came in handy on a cloudy, windy afternoon.
"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.
"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.
By AP National Writer Eddie Pells