Serena Williams admits she is in a tough group as she bids to win her second successive WTA Championship in Turkey.
World number three Maria Sharapova misses the event through injury so eight of the top nine in word rankings are split into two groups of four.
The top two from each group then qualify for the semi-finals.
The end-of-season finale begins in Istanbul on Tuesday and Williams will face Petra Kvitova, Agnieszka Radwanska and Angelique Kerber in her group.
“It’s a tough group,” Williams said. “That’s the thing about this event, you have to be ready from the first match.”
Williams has had a fantastic season when winning 10 titles, including the French Open and US Open and has lost just four matches.
In the other group Australian Open champion Victoria Azarenka, who has beaten Williams twice this year, will face China’s Li Na, Sara Errani of Italy and Serb Jelena Jankovic.
Williams will begin the defence of her WTA Championships crown against world number nine Kerber on Tuesday.
Azarenka starts against Errani, while Radwanska of Poland plays the Czech Republic’s Kvitova.
Link Dafabet