Wimbledon champion Serena Williams was surprisingly knocked out of the Stanford Classic WTA event on Friday after being upset by unseeded Australian Samantha Stosur in the quarterfinals.

The top-seeded Serena, who had a nine-match winning streak snapped, lost 6-2, 3-6, 6-2 to Stosur as the latter, seeking her first WTA Tour title, played a steady game and saved nine of 10 break points in the quarters.

Earlier this year, Stosur had held four match points against Williams in Sydney, but was unable to convert them in a three set defeat.

"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," said the Australian.

Stosur will play the winner of the quarterfinal between fourth seed Jelena Jankovic and France's Marion Bartoli.

Third-seeded Elena Dementieva of Russia also advanced to the semifinals, beating Daniela Hantuchova of Slovakia 6-2, 6-4.

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

(Xinhua News Agency August 1, 2009)