ISTANBUL (AP) - Defending champion Petra Kvitova has withdrawn from the WTA Championships because of a viral illness and will be replaced by Samantha Stosur.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.

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