Defending champion Petra Kvitova has withdrawn from the WTA Championships because of a viral illness and will be replaced by Samantha Stosur.
The WTA gave no details of Kvitova's illness on Wednesday.
The Czech player lost her opening match Tuesday, falling 6-3, 6-2 to Agnieszka Radwanska.
The season-ending tournament brings together the eight top-ranked players in the world. Stosur was the first alternate.