BEIJING (Reuters) - World number one Serena Williams will head to Europe to undergo tests on a left knee injury after she was forced to pull out of the China Open hours before her quarter-final against Sam Stosur on Friday.
The 33-year-old American inflamed the problem late on Thursday after winning her third round match 6-1 1-6 6-2 against Czech Lucie Safarova on the Beijing hard courts.
"I started to feel pain this week. 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," she told reporters on Friday.
"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. I just tried to move as little as possible. So yeah, I felt it mostly when I was serving last night.
"I haven't had time to get an MRI scan yet, but I'll do that and see what the problem is."
The WTA will be hopeful their star attraction will be fit to defend her WTA Finals title in Singapore later this month after recently losing headline act, China's Li Na, to retirement.
Unseeded Australian Stosur faces a semi-final against Wimbledon champion Petra Kvitova, who also had a walkover against a Williams earlier in the tournament when Serena's sister Venus pulled out of their third round match with a viral illness.
Serena, who won her 18th grand slam singles title at the U.S. Open last month, also withdrew from last week's Wuhan Open in China because of illness.
"It's really frustrating, especially having to retire from here," she said, her curtailed and trophy-less Asian swing leaving her downbeat.
"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. To come all this way and not take the title, it's just a waste."
Reporting by Patrick Johnston in Singapore; Editing by Peter Rutherford/Amlan Chakraborty