(CNN) -- Venus Williams has joined her sister Serena in withdrawing from next week's Cincinnati Open, citing a knee problem that could dent her hopes of success at the U.S. Open.
World No. 4 Venus has not played at a tournament since losing in the Wimbledon quarterfinals at the end of June, and said she had experienced soreness while practicing recently.
"During the last three weeks I returned to my normal practice schedule, and last week began to experience pain in my left knee," the 30-year-old American told the Cincinnati hardcourt event's website on Friday.
"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".
Williams, a seven-time Grand Slam winner, played at the U.S. tournament for the first time last year, losing in the third round.
Eight of the world's top-10 players will be present at the Lindner Family Tennis Center, but No. 1 Serena is still on the sidelines due to the cut foot she suffered at a restaurant that will also rule her out of the upcoming tournament in Montreal. CNN
