(CNN) -- Top seed Serena Williams has crashed out of the Bank of the West Classic, but her older sister Venus reached the semifinals after thrashing Maria Sharapova.
World No. 2 Serena Williams is downcast after missing a point during her shock defeat by Samantha Stosur.
Wimbledon champion and world No. 2 Serena was upset 6-2 3-6 6-2 by unseeded Samantha Stosur in the first of Friday's quarterfinals, as the Australian earned a last-four clash with Marion Bartoli.
The eighth-seeded Frenchwoman progressed after continuing her dominance over former world No. 1 Jelena Jankovic, defeating the Serbian fourth seed for the fourth time in their last five encounters after prevailing 3-6 7-6 (7-3) 6-3 at Stanford, California.
Second seed Venus, who lost to her sister in the final at Wimbledon in June, will next face third seed Elena Dementieva after the Russian crushed Slovakia's Daniela Hantuchova 6-2 6-4.
Venus, who needed just 80 minutes to demolish Russian Sharapova, has beaten Dementieva eight times in their past 10 clashes.
But Dementieva, who tested Serena at Wimbledon before losing their semifinal, was in buoyant mood.
"I'm very excited about playing the semifinals in Stanford for the first time," Dementieva told the WTA Tour's official Web site.
"It will be a great experience to play Venus. I'm happy I won in straight sets today, because I can save some energy for tomorrow."
Stosur, who beat then world No. 2 Dinara Safina in Miami earlier this year, was a double-break point down at 2-2 in the second set against Williams but held serve and went on to win 12 of the last 14 points.
"It was really satisfying. I was really pleased how I played. Even if I didn't win today, I'd walk off the court thinking I had played a great match," Stosur said.
"This is an accumulation of what I've been working for the last several years. The fifth game of the third set was the turning point of the match. If I had lost that game, it would have been very difficult to win."
Bartoli saved two match-points at 6-5 down in the second set and rallied from a break down in the third.
"Jelena didn't give me a lot of opportunities in the first and second sets, but once I got to the third set I felt more confident about my chances," the 2007 Wimbledon finalist said.
"Once I saved those match-points at 5-6 and won the game I felt the momentum was on my side, so I felt good going into the tie-break."