Defending champion Petra Kvitova withdrew from the WTA Championships on Wednesday due to a viral illness.

Kvitova went undefeated in last year's tournament, but dropped a 6-3, 6-2 decision to Agnieszka Radwanska in her opening round-robin match on Tuesday.

"Of course I'm sad to withdraw from the WTA Championships. It's a great tournament and it was an honor to be here," Kvitova said.

Samantha Stosur will replace Kvitova for the remainder of the season-ending tournament, which consists of the top eight players in the world.