Dinara Safina withdraws from the WTA Championships with a back injury to hand the year-end top spot to American Serena Williams. Safina's tearful exit after just two games of her first group match against Jelena Jankovic meant 11-times grand slam champion Serena was assured of ending the year top of the pile for the first time since 2002 even before she beat sister Venus later in a midnight thriller. Earlier at the Khalifa Tennis Centre Caroline Wozniacki and Victoria Azarenka, two players who are set to challenge the Williams sisters for major honours, went toe to toe for three hours before Denmark's Wozniacki triumphed 1-6 6-4 7-5.