Dinara Safina was forced to withdraw from the season-ending Sony Ericsson Championships in Doha with a back injury, leaving Serena Williams to end 2009 as the world's number one - a feat the American celebrated with a defeat of her sister Venus.

The 23-year-old Russian was serving at 1-1 in the opening set against Serbia's Jelena Jankovic when she had to retire.

Safina's place at the Australian Open could now be in doubt due to the injury as she said she will have to take at least six weeks off from tennis.

"I've been having pain since (winning) Portoroz (in July). I've been using anti-inflammatories to play with this pain for about three months," she said.

The Russian was replaced in the tournament by compatriot Vera Zvonareva, who will play the remaining two White Group opponents Safina was supposed to play, beginning with Caroline Wozniacki on Thursday.

Serena defeated Venus 5-7 6-4 7-6 (7/4) in two hours and 41 minutes - saving match point along the way - for her second win in two days after her defeat of Svetlana Kuznetsova on Tuesday.

It was Venus' second loss of the tournament, following her defeat by Elena Dementieva, and leaves her on the brink of an exit from the event.

Serena was thrilled to end a year which included victories in the Australian Open and Wimbledon on top spot.

"2009 has been a truly memorable season for me during which I enjoyed some of the biggest wins of my career," she said.

"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."

Earlier, Denmark's Wozniacki defeated Belarus' Victoria Azarenka 1-6 6-4 7-5.

The contest lasted two minutes short of three hours as Wozniacki won her first match of the tournament, inflicting Azarenka's first loss after her defeat of Jankovic.