MADRID World number five Serena Williams has been forced to withdraw from the WTA Championships in Madrid because of a knee injury and her place will be taken by Wimbledon finalist Marion Bartoli.
The 26-year-old American retired after losing the first set of her opening round-robin match against Russian Anna Chakvetadze on Wednesday following treatment to her left knee.
Williams initially thought she may be able to continue playing in the tournament but after further medical tests she announced her withdrawal.
"It was really short visit and I felt really sad. It was really unfortunate," Williams told a news conference.
First reserve Bartoli will step in to the tournament, taking on Williams' result and playing the remainder of her round-robin matches in the yellow group.
The 23-year-old French woman will play her first match against world number one Justine Henin, who has already qualified for the semi-finals after winning her opening two matches.