Poorly Kvitova unable to defend WTA title October 24, 2012 -- Updated 1842 GMT (0242 HKT) Defending champion Petra Kvitova was out of sorts in her match against Agnieszka Radwanska, making 41 unforced errors. Defending champion Petra Kvitova withdraws from WTA Championships with illness 9 Samantha Stosur replaces the Czech in the Istanbul event Serena Williams wins second match in as many days as she beats Li Na of China American earned code violation after smashing her racket in anger after losing service game (CNN) -- Defending champion Petra Kvitova expressed sadness after illness forced her withdrawal from this year's WTA Championships in the Turkish city Istanbul on Wednesday. The Czech, who lost in straight sets to Agnieszka Radwanska of Poland in her opening round-robin match on Tuesday, is suffering from a viral illness. I'm really sad to withdraw from the tournament. It's a great tournament and it was an honor to be here," the 22-year-old told the WTA's official website. I hope I can come back next year. I've never withdrawn from a tournament, this is the first time for me. I don't like to give up, so I'm really disappointed. When I played I thought I was nervous, but normally nerves are gone after the first set, and it was actually happening during all the match. I was shaking all match and wasn't feeling very well. My stomach felt funny. After one rally with Agnieszka I couldn't breathe. Kvitova will be replaced by world no. 9 Samantha Stosur in the season-ending finale for the top eight players. The Australian can still reach the semifinals if she beats second seed Maria Sharapova and Italy's Sara Errani, the French Open runner-up, in her remaining matches in the White Group. "Obviously it's never good when a player pulls out, but I will make the best of the opportunity and look forward to playing," said Stosur. I came to Istanbul straight from Moscow where I played on Sunday, so obviously it is a little bit strange to come here straight after playing a tournament as an alternate. In the opening Red Group match on Wednesday, Serena Williams earned her second victory in as many days with a 7-6 6-3 win over China's Li Na in a match that lasted one hour and 50 minutes. The American's hard fought victory was not without incident as the winner of Wimbledon, the U.S. Open and the Olympics smashed her racket, and so earned a code violation, after serving two successive double faults to lose a service game. "I started out really strong in the first game but then started making errors, and Li started playing her game and playing it well - that's all," Williams told wtatennis.com. I was a little tight and my serve wasn't working at all, maybe towards the end - but that's not how I usually play. The good thing is I can only play better. The 31-year-old, playing her first tournament since winning the U.S. Open, will become the event's oldest champion should she win her third title, with previous successes coming in 2001 and 2009.