Serena Williams starts a strong favourite to win a second successive WTA tennis Championships as the end-of-season finale begins in Istanbul today.
The world number one has only lost four matches and won 10 titles this season, more than any other year of her career.
32-year-old Williams will face Petra Kvitova, Agnieszka Radwanska and Angelique Kerber in the group stages in Turkey.
Eight of the top nine in the world rankings - world number three Maria Sharapova is injured - are split into two groups of four, with the top two in each qualifying for the semi-finals.
Australian Open champion Victoria Azarenka, who has beaten Williams twice this year, heads the other group, which includes China's Li Na, Sara Errani of Italy and Serb Jelena Jankovic