China Open champion Serena Williams spoke of her frustration at withdrawing early from a second consecutive tournament Friday, but said it was too early to say if her season was over.

The top seed in Beijing withdrew from the $5.4 million tournament ahead of her quarter-final match because of a left knee injury.

The world number one was due to play Australian Samantha Stosur after winning her third round match Thursday evening with a bandaged knee.

The power-hitting American is scheduled later this month to play in the WTA Finals in Singapore, the end-of-season finale involving the world's top eight women players. But she refused to say if her participation in that event would be affected by her injury at a press conference in Beijing.

"I don't know yet. I haven't had time to get an MRI scan. I'm going to do all that and see what is the root of the problem, I think," she said.

Williams would have secured the year-end number one ranking if she had retained her title at the final in Beijing Sunday – the fourth time in her career that she has achieved such a feat.

She is also a four-time winner of the end of season finale, and the current champion after defeating recently-retired Chinese star Li Na in last year's final.

"If I feel this way for Singapore, I don't think it's smart for me to play. Other than that, I'm just going to see how it goes," she told reporters Friday.

Williams made it through to the quarters in Beijing after a scare in a back-and-forth game against Czech player Lucie Safarova that she eventually won 6-1, 1-6, 6-2.

Stosur, the 2011 US Open winner and former world number four, will now go straight into the semi-finals. Williams had to be helped from the court as she retired ill during her first set at the inaugural Wuhan Open in China last week.

She said it was "especially" frustrating to withdraw from the China Open.

"It's really frustrating, especially from here. This is a really big tournament for me. I'm defending champ," the 33-year-old said. "So as a defending champ, 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 extremely frustrating. You know, I wanted to be here. To come all this way and not to take the title, it's just a waste."

Her sister Venus, the 16th seed in Beijing, withdrew from the tournament with a viral illness on Thursday.