STANFORD, Calif. — Top-seeded Serena Williams injured her left knee and pulled out of the Bank of the West Classic today, becoming the third singles player on the U.S. Olympic women's tennis team with a knee injury.

She retired while trailing during the second set of her semifinal match against qualifier Aleksandra Wozniak.

Williams' sister Venus and Lindsay Davenport, the other U.S. Olympic singles players, have already pulled out of next week's East West Bank Classic in Carson, Calif., with right knee injuries.

Serena Williams struggled from the start against the 85th-ranked Wozniak and first called for the trainer when trailing 5-2 in the first set. She got the knee heavily taped by the trainer and came out to finish the set, but appeared to struggle to plant on her injured leg.

Wozniak held serve to win the set and won 11 straight points to take a 3-1 lead early in the second. Williams appeared to injure the knee further while stretching for a backhand in the fifth game of the set and retired. She walked off the court and embraced her father, Richard.

Williams underwent surgery on her left knee in 2003 and missed 10 tournaments in 2006 with another injury to the knee. The extent of this injury was not immediately known.

Wozniak will play in Sunday's final against the winner of the night semifinal between sixth-seeded Marion Bartoli and Ai Sugiyama.