Serena Williams lost her fitness battle yesterday, withdrawing from the $600,000 WTA hard court tournament in Los Angeles because of a sore left knee.

"It's a huge disappointment for me to be unable to play the East West Bank Classic in my hometown," said Williams.       

"I've been getting intensive therapy and waiting until the last possible moment to see if I could play," she added.

"Unfortunately after hitting this morning, I knew that wasn't going to be ready for this week," she explained.

Williams, who ranked No 5 in the world, withdrew while trailing against eventual champion Aleksandra Wozniak in the semi-finals of the tournament in Stanford.       

She said this week that an MRI showed inflammation in her knee, which she attributes to a busy competition schedule.

She said that she would now focus on getting fit for the Beijing Olympics in August, which are followed by the final Grand Slam of the season, the US Open.