Venus Williams joined her sister Serena as an injury withdrawal as the seven-time Grand Slam champion pulled out of next week's WTA stop in Cincinnati, Ohio, dpa reported.
Officials said the US player cited a left knee injury. World number one Serena Williams announced her non-participation weeks ago after a mysterious off-court foot injury, which required surgery last month and left her in a walking cast during a recent nightclub visit.
Late withdrawals are nothing rare for the Williams sisters, with Venus to have made her Cincinnati debut after also pulling the plug a year ago.
Her camp said she experienced knee pain but appeared to show no signs of distress during a book-signing tour stop last week in Los Angeles.
A statement from Williams said: "My medical advisors and physio have recommended that I refrain from competition until I am pain- free. Unfortunately, I am not at a point where I can commit to competing next week."