Stanford, CA -- (Sports Network) - Top-seeded Serena Williams bowed out in the semifinals of the $600,000 Bank of the West Classic tennis event on Saturday.

Things did not start off well for last month's Wimbledon runner-up to her older sister Venus. She dropped the first set against Canadian qualifier Aleksandra Wozniak, 2-6. Down 2-5 in the opening frame, and favoring her left leg, Williams called on a trainer to wrap her knee.

It apparently did little good. Trailing 1-3 in the second set, Williams abruptly retired due to an injury to that left knee after only 47 minutes of action.

Williams had breezed past the fifth-seeded Swiss Patty Schnyder 6-3, 6-1 on Friday to gain the semis, and reached the final four in her first appearance at this tournament on the hardcourts at Taube Family Tennis Stadium.

Wozniak, who is making her tournament debut, bested Australian Samantha Stosur 6-2, 5-7, 6-4 to reach the semifinal round. She will face sixth-seeded Marion Bartoli in Sunday's final, who won the other semifinal on Saturday over Japanese veteran Ai Sugiyama.

Bartoli downed Sugiyama in straight sets -- 6-3, 6-3 -- to reach the final. This is Bartoli's sixth appearance at this tournament, but the Frenchwoman had only reached the second round on four previous occasions.

The 2008 Stanford titlist will collect $95,500.