World number one Serena Williams withdrew on Tuesday from next week's Los Angeles Open because of strained left quadriceps.

The thigh injury was diagnosed on Monday after Williams had an MRI scan on her left leg in Los Angeles, forcing her to withdraw from this week's Acura Classic in Carlsbad.

The Wimbledon champion strained her quadriceps over the weekend while practising for the Acura Classic.

Williams also pulled out of last week's Bank of the West Classic in Palo Alto, California because of pain in her left knee.

She will be hoping to recover in time for the U.S. Open which begins on August 25.