STANFORD, Calif. (AP) -- Serena Williams' busy summer schedule caught up with her, making her the latest U.S. Olympian dealing with a knee injury as the Beijing Games approach.
Williams injured her left knee and pulled out of the Bank of the West Classic on Saturday, becoming the third singles player on the U.S. women's tennis team with a knee injury.
Williams retired from her match after losing the first set and falling behind in the second set of her semifinal against qualifier Aleksandra Wozniak.
"It was hurting in practice and I didn't really practice too long because it was hurting," Williams said. "After I got off, it was really swollen. I thought that I have to play really fast."
She lasted only 46 minutes before pulling out of the match trailing 2-6, 1-3. Williams said the injury is different than one that forced her to undergo surgery on her left knee in 2003 and miss eight months. But that knee has bothered her since, forcing her to drop out of the 2004 Athens Olympics and to miss significant time again in 2006.
But Williams had felt healthy most of this year, playing a busy schedule since winning in Miami in the beginning of April. She played 26 matches in a three-month span that went through her loss in the Wimbledon final to big sister, Venus.
Then she returned home and played for the Washington Kastles of the World Team Tennis league before arriving at Stanford for her debut in this tournament. Williams wouldn't blame her decision to play team tennis on her latest setback.
"You know that risk going into it," she said. "It is what it is. I can't blame that. I just think in general I've been playing a lot of tennis since Miami - especially for me."
Venus Williams 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., due to right knee injuries. Serena Williams said she still plans to go to Carson, but will likely have an MRI before then and could choose to rest up for the Olympics next month.
"I do plan on obviously being at the Olympics," she said. "That's my main goal."
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 and came out to finish the set, but appeared to struggle to plant on her injured leg.
"It's unfortunate that she pulled out," Wozniak said. "I think in the middle of the first set she started looking at her leg. I didn't know if it was her ankle bothering her. But at the end it was her knee."
Wozniak held serve to win the set and won 11 straight points to take a 3-1 lead early in the second. Williams then aggravated the knee while stretching for a backhand in the fifth game of the set and retired from the match. She walked off the court and embraced her father, Richard.
"I actually thought it felt better once I got it wrapped," Williams said. "But Wozniak ke pt moving me. I was hoping she wouldn't but I guess she knew better. That didn't help at all."
Wozniak will play in Sunday's final against the winner of the night semifinal between sixth-seeded Marion Bartoli and Ai Sugiyama.
It's been quite a week for Wozniak, who needed to win three matches in the qualifying tournament just to make it into the main draw. She then beat eighth-seeded Francesca Schiavone in three sets in the first round, before knocking off Sybille Bammer and Samantha Stosur to make it to the semifinals.
This is just the second career final for Wozniak, who made it to a lower-tiered championship match in Morocco last year. Wozniak, who entered the week ranked 85th in the world, should be close to reaching her goal of being a top 50 player by next week.
"I'm really happy with the way I played this week, coming out of the qualifying is not easy," Wozniak said. "It's a lot of matches in one week."