Serena out, Marion in
World number five Serena Williams has withdrawn from the WTA Championships in Madrid because of a knee injury and her place will be taken by Wimbledon finalist Marion Bartoli.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 a really short visit and I felt really sad. It was really unfortunate," Williams told a news conference.
First reserve Bartoli will step into the tournament, taking on Williams' result and playing the remainder of her round-robin matches in the yellow group.
The 23-year-old Frenchwoman will play her first match against world number one Justine Henin, who has already qualified for the semifinals after winning her opening two matches.
Williams began the year at 95 in the world having been restricted to four events in 2006 because of an injury to her left knee.
A shock victory in Australia, the eighth grand slam title of her career, sparked her return to the elite which was confirmed by victory in Miami in April.
However, the American's season was also punctuated by a series of injury problems.
A groin strain forced her retirement during her opening match in Charleston, an injured thumb hampered her progress at Wimbledon while a muscle strain forced her to retire in the first round in Zurich last month.
Williams was optimistic she would be back fit to defend her Australian Open title at the start of next year.
"Count Serena for Australia, the Fed Cup and all the other tournaments," she said.