Reigning champion Petra Kvitova withdrew Wednesday from the season-ending WTA Championships tournament due to bronchitis.

The $4.9 million tournament kicked off on Tuesday, when the Czech world No. 6 made 41 unforced errors in her 6-3, 6-2 defeat to Poland’s Agnieszka Radwanska, a performance she blamed on nerves.

But Kvitova, who has asthma, admitted to health problems Wednesday.

“I am very sorry to tell you that I have to withdraw from the WTA Championships,” said the 22 year old, who defeated Victoria Azarenka in last year's final, in comments on her Facebook page.

“Unfortunately, I am suffering from bronchitis and after consulting with the WTA medical team I have decided that I am unable to continue here in Istanbul.”

Australia’s Sam Stosur, ranked ninth in the world, will now replace the 22 year old to face Russia’s Maria Sharapova and Sara Errani of Italy in the white group.