World number 10 Andrea Petkovic pulled out of the Australian Open with a stress fracture in her back on Wednesday, becoming the second leading women's withdrawal from the year's opening grand slam. The exciting German joined seven-time grand slam champion Venus Williams, who has an autoimmune disease, on the sidelines, while Kim Clijsters and Serena Williams are among the other top players who are also battling injuries. Petkovic said the stress fracture in her lower back came to light in a scan after her straight-sets loss to Poland's Agnieszka Radwanska in the second round of the Sydney International on Tuesday. "I've been having some back problems for the last three or four months," she said in a WTA statement. "I've been struggling with it, but I've always believed it was going to be fine and so I pushed through the pain. "The last two weeks in Australia I've been having bigger problems and I could only play for 30-45 minutes without pain." She added: "The only thing I can do right now is rest and hope that the bone will heal as fast as possible." Petkovic said she was "positive" she would make a complete recovery but would have to be patient. She will return to Germany later on Wednesday for further tests. "I'm positive this is going to be fine. I just need to take the time off to get better," he said. "With this type of injury I'd rather take an additional week off, than start back a day early, and I need to be patient." On Monday, Venus Williams announced her withdrawal as she struggles to recover from Sjogren's syndrome, while her sister Serena was forced out of last week's Brisbane International with a sprained ankle. Defending champion Clijsters also pulled out of the Brisbane event when she suffered a hip injury during her semi-final with Daniela Hantuchova. However, the Belgian says she will be fit for Melbourne. Russian triple grand slam winner Maria Sharapova was another withdrawal from the Brisbane event with an ankle injury, but said she expected to be ready for the Australian Open. The men's draw is also affected by injuries with former champions Roger Federer and Rafael Nadal among the players contending with fitness problems.