(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, while Serena Williams and Maria Sharapova reached the semifinals of the season-ending event.
Kvitova, 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 Czech 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 I've never withdrawn from a tournament, this is the first time for me.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 elite eight-woman lineup.
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.
Second-ranked Sharapova went through to the last four after coming from behind to overcome Radwanska 5-7 7-5 7-5 in a match lasting three hours and 12 minutes that finally finished after 2 a.m. local time.
The Russian, who won the tournament as a 17-year-old in 2004, faced defeat at 4-2 down in the second set to follow up her opening win over Errani.
"I was a little bit frustrated early on. I wasn't doing the right things and she was doing all the right things, and making me make mistakes," said Sharapova, who hit 66 winners to Radwanska's 18 but also led in unforced errors 64 to 19.
"I was injured last year and didn't play very well, so to be in the semifinals this year makes me very proud, and especially given the way I fought tonight."
In the Red Group, world No. 1 Victoria Azarenka stayed on course to finish the year as the top-ranked player -- but only just as she took over three hours to see off the challenge of Angelique Kerber.
The Belorussian saved two match points against the German to win 6-7 7-6 (13-11) 6-4 and now needs just one more win to finish on top, and so deny Sharapova the chance to end the year as No. 1.
"On the match points I just thought I had to go for it -- she wasn't going to miss, and I didn't want to wait for her to make a mistake," Azarenka said.
"I was so determined to make it happen. I wanted to take destiny into my hands -- if I missed those shots, it was going to be up to me, not up to my opponent."
In the earlier Red Group match on Wednesday, Williams earned her second straight-sets 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 said.
"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.