Petra Kvitova of the Czech Republic wipes her forehead after missing a shot against Petra Martic of Croatia at the Pan Pacific Open tennis tournament in Tokyo September 25, 2012.??[Photo/Agencies]

ISTANBUL - Defending champion Petra Kvitova has withdrawn from the WTA Championships due to a viral illness and will be replaced by Samantha Stosur, tournament officials announced on Wednesday.

The world number six was able to play her first match in the White group on Tuesday but was toppled 6-3 6-2 by Poland's Agnieszka Radwanska in a contest that saw the Czech commit a whopping 41 unforced errors.

Australian Stosur will complete the other matches Kvitova was due to play in the four-woman round robin group against Russia's Maria Sharapova and Italy's Sara Errani.

The WTA Championships features the world's top eight women with the Red group comprising Victoria Azarenka of Belarus, American Serena Williams, Angelique Kerber of Germany and Li Na of China.

Kvitova is scheduled to represent her country in the Fed Cup final against Serbia in Prague next week but her participation in that event has yet to be confirmed.

Serena Williams beat?Li Na?of China 7-6, 6-3 at the season-ending WTA Championship here on Wednesday