Defending champion Petra Kvitova has pulled out of the WTA Championships in Istanbul due to a viral illness.
The world number six from the Czech Republic will be replaced in the White Group by Australian Samantha Stosur, the WTA announced on their official Twitter account.
Kvitova, who beat Victoria Azarenka in last year's final, lost her opening match of the tournament on Tuesday, going down 6-2 6-3 to Poland's fourth seed Agnieszka Radwanska.
The defeat ended Kvitova's 25-match winning streak on indoor hardcourts and earned Radwanska her first win in four career meetings with the Czech.
Stosur, the world number nine, joins Radwanska, Maria Sharapova and Sara Errani in the White Group, with Azarenka, Serena Williams, Li Na and Angelique Kerber in the Red Group.
