Serena Williams will finish the year as world number one after Dinara Safina pulled out of the Sony Ericsson Championships in Qatar.
The Russian was left in tears when she was forced to retire with a lower back injury at 1-1 in the first set of her clash with Jelena Jankovic
The 23-year-old subsequently announced her withdrawal from the season-ending event in Doha and is now expected to take six weeks off.
She revealed that she had been struggling with the problem for three months but carried on playing to try and remain on top the WTA Tour rankings.
Safina is hoping surgery will not be required but a lengthy period on the sidelines will put her participation in next year's Australian Open in doubt.
"I've been having pain since (winning) Portoroz (in July)," she said. "I've been using anti-inflammatories to play with this pain for about three months."
Vera Zvonareva takes Safina's place in the event and will face Caroline Wozniacki in the opening match on court on Thursday.
Serena, who began the tournament 155 points behind in the rankings, won her opening match against Svetlana Kuznestova on Tuesday.
The American then made it two wins from two with a 5-7 6-4 7-6 (7-4) result against her sister Venus on Wednesday.
"2009 has been a truly memorable season for me during which I enjoyed some of the biggest wins of my career," she said afterwards.
"Capturing the year-end world number one ranking is a huge accomplishment for me and I am thrilled that all the hard work has paid off."