STANFORD: 1 Australian Samantha Stosur shocked top seed Serena Williams while second seed Venus Williams overpowered Maria Sharapova in WTA Bank of the West Classic quarter-final matches on Friday.

Stosur snapped Serena’s nine-match win streak with a 6-2, 3-6, 6-2 win to book a semi-final berth against French Marion Bartoli, who upset Serbian Jelena Jankovic 3-6, 7-6 (7/3), 6-3.

Venus Williams defeated Russian star Sharapova 6-2, 6-2 to book a semi-final date with Russian Elena Dementieva, who ousted Slovakia’s Daniela Hantuchova 6-2, 6-4.

Serena Williams, the 11-time Grand Slam champion who won Wimbledon and the Australian Open this year, took both prior meetings against Stosur but was forced to save four match points in a three-set win in January. Stosur denied Serena on 9-of-10 break points and deprive the 34-time WTA champion’s comeback bid at the $700,000 hardcourt tournament.