Second-seeded Serena Williams lost her fitness battle on Tuesday, withdrawing from the $600,000 WTA hardcourt tournament in Los Angeles because of her sore left knee.
"It's a huge disappointment for me to be unable to play the East West Bank Classic in my hometown of LA," said Williams, who said on Monday she hoped to play despite the injury.
"I've been getting intensive therapy and doing everything in my power to get my knee in shape for this week, 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."
Williams, ranked No.5 in the world, withdrew while trailing against eventual champion Aleksandra Wozniak in the semi-finals of the tournament in Stanford, California, on Saturday.
She said this week that an MRI showed inflammation in her knee, which she attributed to a busy competition schedule.
The American, who finished runner-up to elder sister Venus at Wimbledon earlier this month, said 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.
"I'm working hard to be ready for the Olympics and US Open," said Williams, who had a first-round bye in Los Angeles and will be replaced in the draw by lucky loser Melanie South of Britain.