Reigning Wimbledon and US Open champion Serena Williams has withdrawn from the China Open because of flu while her sister Venus has also pulled out, citing a back injury, organisers said on Friday. Serena, who confirmed her place among the tennis greats this year by raising her Grand Slam haul to 15 and winning Gold at the London Olympics, pulled out late on Thursday, just two days before the start of the first round. It is a big blow to the organisers of the Beijing event as the world number four -- who has already qualified for the season-ending WTA Championships in Istanbul, Turkey, next month -- is one of the tour's most bankable stars. "We would have loved for Venus and Serena to play the China Open this year, but unfortunately players get injured and sick and are unable to compete," Charles Hsiung, a director of the tournament, said. "We hope that they both recover quickly and look forward to them returning to the National Tennis Stadium in 2013," he added. Venus, the elder sister and a former world number one, has had a difficult year and currently lies 41 in the world rankings. "I was looking forward to playing at the China Open this year, but am currently working through a minor back injury and will unfortunately need more time to recover," she said. The WTA China Open starts on Saturday with Agnieszka Radwanska of Poland defending her title after against a host of top players, including world number one Victoria Azarenka, Russian Maria Sharapova and home favourite Li Na.