Serena Williams insists she will be fit to defend her Australian Open title despite withdrawing from the Champions Challenge tournament in Hong Kong.
Williams pulled out of her clash with China's Zheng Jie and said: "I've developed some swelling in my knee and was advised that I shouldn't play."
There was also bad news for Williams' sister Venus, who lost 6-3 6-1 to Lindsay Davenport in the semi-finals.
Venus, 10th in the world, complained of discomfort in the back or pelvis.
"I couldn't get my feet in position or move very much," she said.
Serena, who had a series of ankle and knee injuries last year, insisted she would be fit for the Australian Open, which starts on 16 January.
"I don't know what to say. I feel really, really awful, it's ruined my whole day to say the least; my whole week," she added.
The seven-times Grand Slam-winner was soundly beaten in her first game in Hong Kong on Thursday by Russian Elena Dementieva 6-3 6-1.
That was her first competitive match since she suffered an embarrassing first-round defeat at the China Open in September.
Belgian Kim Clijsters will play Russia's Elena Dementieva in the other Hong Kong semi-final.