Citing health setbacks as the main reason, former No. 1 player Ana Ivanovic, 29, announced her retirement from tennis on Wednesday.
Serena Williams will meet Caroline Wozniacki in the semifinals of the WTA Finals while Simona Halep will take on Agnieszka Radwanska.
Simona Halep beat two-time defending champion Serena Williams 6-0, 6-2 at the WTA Finals on Wednesday, with the American's two games won equaling her career worst.
Caroline Wozniacki credited her training for next month's New York Marathon as a key factor in her victory over Maria Sharapova in a grueling three-set match in the WTA Finals on Tuesday.
Serena Williams says comments by the head of the Russian Tennis Federation referring to her and older sister Venus as "brothers" were bullying, sexist and racist, and that she supported the one-year suspension imposed by the WTA against the official.
Wimbledon champion Petra Kvitova is out in the third round of the US Open.
