Wimbledon champion Serena Williams has crashed out of the Bank of the West Classic.

Top seed Williams lost 6-2 3-6 6-2 to Australia’s Samantha Stosur in the quarter-finals of the event at Stanford.

Stosur - who is seeking her first WTA Tour title - will play Marion Bartoli for a place in the final, after the eighth-seeded Italian defeated fourth-seeded Serbian Jelena Jankovic 3-6 7-6 6-3.

Venus Williams - beaten by her sister in the Wimbledon final last month - reached the semi-finals with a straight sets defeat of Maria Sharapova.

Williams won 6-2 6-2, accepting her second match point to wrap up victory in 80 minutes and set up a meeting with third-seeded Elena Dementieva, who beat Daniela Hantuchova 6-2 6-4.