PARIS — Early on, Serena Williams knocked dirt from her shoes by angrily whacking them with her racket, as if punishing the clay that keeps tripping her up at the French Open.

The top-ranked Williams stumbled in the quarterfinals Wednesday, squandering a match point and losing to Australian spoiler Samantha Stosur, 6-2, 6-7 (2), 8-6.

A 12-time Grand Slam champion, Williams won her only French Open title in 2002 and hasn't been to the semifinals since 2003.

The upset was the second in a row for the No. 7-seeded Stosur, and the third in as many days at Roland Garros. Stosur ended four-time champion Justine Henin's Roland Garros winning streak at 24 matches in the fourth round.

On the men's side Tuesday, top-ranked Roger Federer lost to Robin Soderling.

Then Williams made her exit, with stretches of brilliant tennis by Stosur hastening the departure. The Australian, long regarded as a doubles specialist, used her forceful forehand to build a lead, winning 17 consecutive points during one stretch.

Williams mounted one of her patented comebacks, and as the tension built in an error-filled third set, she needed only one point in the 10th game for the victory.

Her forehand sailed an inch long.

Stosur then regained her early form. She hit consecutive cross-court winners to break for a 7-6 lead, then served out the victory, hitting service winners on the final three points.

"I've calmed down a little bit since walking off the court," a smiling Stosur said 90 minutes after the match. "But I'm pretty happy with myself."

It was Williams' first Grand Slam loss since her meltdown in the semifinals of the U.S. Open last September against Kim Clijsters. This time she directed any anger only at herself — and her shoes.

Stosur, a semifinalist for the second year in a row, will play Thursday against No. 4 Jelena Jankovic, who beat unseeded Yaroslava Shvedova 7-5, 6-4. Jankovic, who rallied three times from a service break down in the second set, also reached the semifinals in 2007 and 2008, then lost each time.

With sunshine returning at Roland Garros, Stosur wore her distinctive sunglasses and looked especially spiffy at the start, embracing the role of underdog.

In contrast, Williams often seemed hesitant, indecisive and on the defensive, pinned deep by Stosur's heavy topspin forehand and slice backhand. Williams hit one feeble backhand that barely reached the bottom of the net, took an awkward swing at an overhead and flubbed a forehand putaway in the forecourt.

"I didn't want to let her try and dictate the points," Stosur said. "I tried to do that straight back to her. You definitely have to go after her."

Williams hit 13 aces but also nine double-faults and committed 46 unforced errors to 24 for Stosur.

Williams was serving at 2-3, 30-all in the first set when her game began to unravel. She committed unforced errors on the next two points to lose serve, and Stosur won the next three games at love, a shocking streak against the world's No. 1 player.

Stosur looked nervous for the first time serving for the match at 5-3 in the second set. She double-faulted for the first time, hit the net post with a forehand and fell down chasing a ball on break point.

Several tentative shots cost her in the tiebreaker, and the ever-resilient Williams evened the match.

"In the third set, I just tried to hang in there, waited for another opportunity, and I took it," Stosur said.

At the finish, she hit a flurry of winners that had the center court crowd roaring. When her final serve didn't come back, she raised her arms in triumph and took off her glasses, giving a fans a good look at the surprise semifinalist.

But perhaps not that surprising: Stosur has the most wins on clay this year on the women's tour, with a record of 19-2. A two-time Grand Slam champion in women's doubles, she's ranked a career-best No. 7.

The Williams sisters were seeded No. 1 and 2 at Roland Garros, but the family has only that single French Open title won by Serena. Venus was eliminated in the fourth round by Nadia Petrova.

The other semifinal will be between No. 5 Elena Dementieva and No. 17 Francesca Schiavone, the first Italian woman to reach the final four at Roland Garros since 1954.

In the men's quarterfinals Tuesday, Soderling played the role of French Open spoiler yet again. The big-swinging Swede ended Federer's record streak of reaching 23 consecutive major semifinals by winning 3-6, 6-3, 7-5, 6-4.

A year ago, Soderling beat four-time champion Rafael Nadal to clear Federer's path to his first French Open title and a career Grand Slam.

If this keeps up, Soderling's victories won't be upsets anymore.