Australia's [Images] Samantha Stosur [Images] employed a huge serve, hard forehand and deft backhand slice to upset Wimbledon [Images] champion Serena Williams [Images] 6-2, 3-6, 6-2 in the quarter-finals of the Stanford Classic on Friday.

Stosur had held four match points against Williams in Sydney [Images] earlier this year, but was unable to convert them in a three set defeat, but on Friday, was the much more consistent player.

"I knew I had my chances in Sydney and I wanted to get in the same position, and this time I got over the line," Stosur said.

After fighting off two break points at 2-2 in the third set, Stosur then broke the second-ranked Williams to love and went ahead 3-2 when Williams erred on an easy backhand.

The Australian then won the next three games in routine fashion and took the contest when Williams double faulted.

Stosur will play the winner of the match between fourth seed Jelena Jankovic [Images] and France's [Images] Marion Bartoli [Images].

Third seed Elena Dementieva [Images] of Russia [Images] also advanced with a 6-2, 6-4 victory over Slovakia's Daniela Hantuchova [Images].

Dementieva will play the winner of the match between second seed Venus Williams [Images] and Maria Sharapova [Images].

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