Dinara Safina has withdrawn from the Sony Ericsson Championships, leaving Serena Williams to end 2009 as the world's number one.
The 23-year-old Russian was serving at 1-1 in the opening set with Serbia's Jelena Jankovic today when she had to retire.
Safina's place at the Australian Open could now be in doubt due to the injury.
In today's opening match, Denmark's Caroline Wozniaki defeated Belarus' Victoria Azarenka 1-6 6-4 7-5.
The contest lasted two! minutes short of three hours as Wozniaki won her first match of the tournament, inflicting Azarenka's first loss after her defeat of Jankovic yesterday.
