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Beijing, China (Sports Network) - Serena Williams and Venus Williams have pulled out of the China Open, which commences Saturday in Beijing.

The China Open is considered a mandatory event on the WTA circuit.

Serena is battling the flu, while her fellow former world No. 1 sister Venus is out with a back injury.

The high-flying Serena was the China Open champion in 2004.

The currently fourth-ranked Serena just enjoyed one of the best summers in the history of women's tennis, as she captured Wimbledon, an Olympic gold medal and the U.S. Open.

The 41st-ranked Venus has played sparingly since being diagnosed last year with Sjogren's syndrome, an autoimmune disease that causes fatigue and joint pain.