Williams sisters out of China Open Serena Williams, shown after wining the women's singles title at ther 2012 U.S. Open, on Friday withdraw from the China Open because of the flu. 28 (UPI) -- Serena and Venus Williams both pulled out of the China Open Friday, citing illness or injury. The China Open is a Premier-level event on the WTA schedule and is one of four "mandatory" tournaments for 2012. Serena Williams is said to have the flu while Venus Williams has a back injury. They also withdrew from the event in 2011. Serena Williams is No. 4 in the world after a summer in which she won the Wimbledon Championship, the gold medal at the Olympics and the U.S. Open. Venus Williams has had her play limited by Sjogren's syndrome, an autoimmune disease that causes fatigue and joint pain. She is 41st in the rankings. The China Open begins Saturday.