Serena Williams she was forced to withdraw from the China Open 2014 with a left knee injury.

Williams is the defending champion at the Premier Mandatory-level event and could have clinched the year-end No.1 ranking by winning the title again this week. 

That race will wait another day now. Williams met the press and talked to them about the injury. "I started to feel pain this week," she commented. "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." And the World No.1's next steps? "I'm probably going to go to Europe and see my doctors there," she said. "They have a really good system there." Williams was also asked her feelings about her Asian swing this year - she had to retire in her first match in Wuhan due to a viral illness, and now the left knee injury withdrawal here in Beijing. "It's really frustrating, especially having to retire from here," she said. "This is a really big tournament for me. I'm defending champ. Also it's one of the big four. You always want to do your best to defend your title. That's why I played yesterday - I've got to at least give it a chance. I've done well here. This is my title. So more than anything, it's just extremely frustrating not to win this week. "To come all this way and not take the title, it's just a waste."