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.

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 said.

"It throbs just sitting, standing. I felt it mostly serving because I'm landing on my left knee. That was really killer for me."

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.

Source: Reuters