"It's one of the biggest, if not the biggest win of my career," Stosur said of her 6-2, 3-6, 6-2 victory over the Wimbledon champion. "We don't always play our best tennis every single day. Maybe she didn't play her best and I played very well."
Despite having four match points, Stosur lost her previous meeting with Williams in three sets in Sydney earlier this season. She also faced the Williams' sisters in the doubles finals at Wimbledon.
"She had a lot of lucky shots; she's a good framer," said Williams, who has won 34 Tour singles titles, including 11 Grand Slam titles. "But it's obviously all talent. She's mastered that."
Stosur saved nine of 10 break points on Friday and played a steady game against the top-seeded Williams, who had a nine-match winning streak snapped.
"She played well and didn't do anything bad," Williams said. "She went for broke and struck all her balls as hard as she could. She never lets you get into a rhythm."
"At the end of the day you have to go out and play well," Stosur said. "I'd like to take it one more step. I'd love to make it to another final."
Sharapova, who returned to the court earlier this year after a nine-month layoff, had 30 unforced errors in her loss.
"Against Venus you have to go more for your shots because you don't expect her to make the errors," Sharapova said. "She did many things a lot better than I did. My goal was to create my own opportunities and I didn't do that."
Venus Williams had 14 winners, including three aces, against 17 unforced errors.
"I can't complain too much," Williams said. "I could have played a little more consistently but I just really want to keep elevating my game through the week."
Bartoli, ranked No. 15, beat Jankovic for the fourth straight time and improved to 10-3 against her.
"The first time I played her she beat me very easily," Bartoli said. "She ran me all over the court and physically wore me down. Once I worked myself into better shape, I was able to run much better."
Jankovic, who lost two match points in the second set, said she could still take some positive out of the loss.
"In the third set I was physically drained and didn't have enough energy to finish the match," she said. "But I played quite well in the first set and I can analyze what was good and where I can improve."
"I felt like it was hot out there and I was struggling with my serve," Dementieva said. "But I was happy to win in two sets, save my energy for the semifinal, and very excited to play my game."
The Bank of the West Classic and Stanford University agreed to a three-year contract earlier that will keep the longest-running women-only professional tennis tournament in the world at the Taube Family Tennis Center through 2012.