Defending champion Petra Kvitova withdrew from the WTA Championships in Istanbul, Turkey, on Wednesday because of a viral illness and will be replaced by Samantha Stosur.

Kvitova said she wasn't sure what she was suffering from, but that she had a sore throat and stomach problems.

She was scheduled to see a doctor later Wednesday.

The Czech player lost her opening match Tuesday, falling 6-3, 6-2 to Agnieszka Radwanska.

Kvitova said she her been shaking throughout the match.

The season-ending tournament brings together the eight top-ranked players in the world.

Stosur was the first alternate.

Kvitova said she was hoping to recover in time to play in the Fed Cup final next week against Serbia.

"I try my best to be ready for the Fed Cup," she said. "It's final at home and I really want to play in front of our crowd, my family and friends and try to win the Fed Cup title again.

"I will try my best to be ready for the weekend."