Defending champion Serena Williams has pulled out of the China Open through injury. The top-ranked Williams had been due to face Australian Samantha Stosur in the quarter-finals on Friday but has withdrawn due to an issue with her left knee.Williams, 33, beat Lucie Safarova 6-1 1-6 6-2 in the third round on Thursday evening but did so with heavy strapping on the injury and then skipped her post-match press conference because of swelling in the knee. “I started to feel pain this week,” she told the WTA Tour’s official website. “It didn’t feel really well last night at all. I almost didn’t go on the court – I did at the last minute. I think it made it a little worse, to be honest. “It throbs just sitting, standing. I felt it mostly serving because I’m landing on my left knee. That was really killer for me. Moving was okay – I didn’t do a tremendous amount of moving last night. I just tried to move as little as possible. So yeah, I felt it mostly when I was serving during the match. “I haven’t had time to get an MRI scan yet, but I’ll do that and see what the problem is.” Williams retired from the Wuhan Open a week ago during her first-round match with a viral illness. Her sister, Venus, also withdrew from the China Open earlier this week, citing a virus. Stosur, meanwhile, will next face third seed Petra Kvitova after the Czech took an hour and 35 minutes to dispatch Roberta Vinci 7-6 (7/2) 6-4 on Friday.