Istanbul: Defending champion Petra Kvitova has withdrawn from the WTA Championships because of a viral illness and will be replaced by Samantha Stosur.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. (Read: Radwanska beats Kvitova)
The season-ending tournament brings together the eight top-ranked players in the world. Stosur was the first alternate. ( Related read: Kvitova to lead Czechs in Fed Cup final)