Serena Williams withdrew from the Sony Ericsson Championships on Thursday after being forced to retire from a match the previous day against Anna Chakvetadze with a left knee injury. The fifth-ranked Williams had lost the first set 6-4 in Wednesday's match before having an MRI scan. It's the same knee that underwent surgery in 2003 and also forced her to miss 10 tournaments last year. "I felt it maybe a little bit yesterday, but it wasn't too intense," Williams said. "I had a really short practice yesterday, and the day before I had a short practice. It's really unfortunate."

Wimbledon finalist Marion Bartoli replaced Williams and was to play defending champion Justine Henin later Thursday. Bartoli assumed Williams' one loss for the remainder of the round-robin tournament.

Williams, who had strips of material on both knees, was treated by a doctor before the ninth game at 4-4. The 2001 champion played on, but the seventh-ranked Russian had little trouble closing it out.

"I was hoping that (the bandages) would work out for me," Williams said.

Williams, who had reached the Sony Ericsson Championships final in her three previous appearances, said defending her Australian Open title in January will be her focus to return.

This has been another frustrating year for Williams with injuries.

After winning her eighth career Grand Slam at the Australian Open, Williams has withdrawn from nine tournaments due to illness, a groin injury, a thumb sprain and an adductor strain.