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Angelique Kerber, Petra Kvitova and Sara Errani have all booked their slots in the season-ending WTA Championships.
Results at this week's China Open mean the trio cannot be caught in the race to finish in the top eight.
Defending champion Kvitova said: "I'm so happy to have qualified again for the WTA Championships - it's always an honour to defend a title, and to do so in Istanbul makes it extra special.
"It has been a challenging year, so to finish among the top eight is really rewarding. I had a great time in Istanbul last year and I look forward to returning and trying to play my best for the fans in Turkey."
Wimbledon semi-finalist Kerber has won two WTA titles in 2012, while Errani has four and also made her first Grand Slam final appearance at the French Open, losing to Maria Sharapova.
Sharapova, Victoria Azarenka, Serena Williams and Agnieszka Radwanska had already qualified.
Li Na, Samantha Stosur, Marion Bartoli and Caroline Wozniacki will now battle it out for the final place in the field.
The event takes place in Istanbul from October 23-28.