Already qualified for the semifinals, top-seeded Caroline Wozniacki beat Slovakia's Daniela Hantuchova 3-6, 7-6 (4), 6-4 on Thursday to make it three wins from three at the season-ending Tournament of Champions.
Hantuchova's aggressive start paid off in the first set, but the former top-ranked Wozniacki gradually took control in the second.
"It was a great match and I am really happy for my victory. Tomorrow I will enjoy my day off," Wozniacki said.
Italy's Roberta Vinci earned her second victory of the group stage by defeating Hsieh Su-Wei of Taiwan 6-1, 6-2, and also advances to the semifinals from Group Serdika.
In the other group, Nadia Petrova beat doubles partner Maria Kirilenko 3-6, 7-6 (4), 6-3.
Following her defeat, Kirilenko was forced to pull out of the tournament with an upper respiratory illness, meaning that Petrova and Bulgaria's Tsvetana Pironkova qualify for the semifinals from Group Sredets.
The pair will play each other on Friday to determine who finishes first and second in the group. China's Zheng Jie will play alternate Sofia Arvidsson, though neither can advance.