Defending champion Petra Kvitova has withdrawn from the WTA Championships in Istanbul with a viral illness. Australian Samantha Stosur will replace her.

The sixth-ranked Kvitova has been battling various illnesses on and off all year, and on Tuesday she lost her opening match to Agnieszka Radwanska, 6-3, 6-2. While the Czech teared up in her press conference afterward and complained of being nervous and unable to move, she made no mention of being sick.

“Yesterday when I played I thought that it's nerves, but normally nerves are gone after first set,” Kvitova said. “I was shaking all match and I wasn't feeling very well. My stomach felt really funny. After one rally with Agnieszka I couldn't breathe, so that's why maybe it looks so easy and I had a lot of errors then. After that I felt still the stomach not really well. I had the diarrhea yesterday also in the morning. ... I never withdraw from a tournament. This first time for me, so I don't really experience this kind of press conferences. I don't like to give up, so I'm feeling really disappointing. I hope that Sam will win some matches for me.”

Stosur, who is the first alternate, will join the “White” round robin group, which includes No. 4 Radwanska, No. 2 Maria Sharapova, and No. 8 Sara Errani. Stosur will play Errani, who lost to Sharapova yesterday, on Thursday. In order for Stosur to reach the semifinals, she will have to beat both Errani and Sharapova and hope that Radwanska either wins her next two matches (against Errani and Sharapova) or loses them both. The first tiebreaker is number of matches won and the second is number of matches played.  
Kvitova is still scheduled to play next week's Fed Cup final in Prague against Serbia, a competition she has said is extremely important to her. The Fed Cup final begins on November 3.

“I try my best to be ready for the Fed Cup,” Kvitova 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.”