(CNN) -- Petra Kvitova dumped Caroline Wozniacki out of the WTA Championships with a straight sets victory that sealed her place in the final four.
Kvitova, the 2011 Wimbledon champion, inflicted Wozniacki's second defeat in Istanbul, winning 6-4 6-2 to go top of the Red Group.
World number one Wozniacki, who called the trainer on during her match after complaining of feeling sick, is rock bottom of the group after playing all three of her round robin games.
After her victory Kvitova told CNN she was delighted to make the final four: "It's nice when I win and I'm happy to be in the semi-final. Istanbul is a great place and it is great experience for me.
"I have had a great season and it's very nice to play here -- it's like something new for me as I'm still learning. I am enjoying every match I play and I'm trying not to think about winning here."
Wozniacki told reporters: "I tried my best, but my body didn't want to do the things I asked it today. What I told my brain didn't go to my body.
"It's just unfortunate that my body has been feeling tired. To get sick now is not the best time if you want to beat the top players."
Czech Kvitova faces Agnieszka Radwanska on Friday -- a match that will determine who grabs the second qualification spot in the red group.
Radwanska, from Poland, currently occupies second spot after she saved three match points to beat Russia's Vera Zvonareva 1-6 6-2 7-5. The Pole can make sure of her place in the semis as long as she wins a set in her clash with Kvitova.
In the white group, Victoria Azarenka sealed her spot in the last four after beating China's Li Na 6-2 6-2.
The Belarusian added to her victory over Australia's Sam Stosur to ensure her meeting with Marion Bartoli, who replaced the injured Maria Sharapova, is academic.
Azarenka told the WTA's official web site: "Li Na has such a good return and likes to dominate from the beginning, so I tried to serve well and take that away from her.
"The crowd here in Istanbul just makes me want to play well and show them some good tennis. They're a fantastic crowd and make me want to play really well."
The final qualification spot in the white group will go to the winner of Friday's clash between Li Na and Stosur.