Wozniacki, the top seed for the tournament, battled seventh-seeded Daniela Hantuchova for 2 hours, 45 minutes before claiming a 3-6, 7-6 (7-4), 6-4 decision.
Both players won 58 percent of the points on serve and Hantuchova won more points (109-107) overall. But Wozniacki saved six break points in the third set and had a break in the ninth game to set up the win.
That left Wozniacki 3-0 in Group Serdika play while Hantuchova is 0-2.
The group's second semifinals slot was taken by fourth-seeded Roberta Vinci who ripped fifth-seeded Hsieh Su-Wei 6-1, 6-2 in 53 minutes. Vinci is 2-1 in round-robin play; Hsieh 0-2.
The semifinalists in Group Sredets were determined as second-seeded Nadia Petrova barely outlasted third-seeded Maria Kirilenko 3-6, 7-6 (7-4), 6-3. Petrova converted all three of her break chances in the third set of the 2-hour, 48-minute match to get to 2-0 in the group.
Kirilenko and eighth-seeded Tsvetana Pironkova are 1-1 but Kirilenko, citing an upper respiratory illness, withdrew. Zheng Jie (0-2 in the group) will play alternate Sofia Arvidsson Friday but neither can advance to the semifinals, boosting Petrova and Pironkova into weekend play.
Petrova and Pironkova will meet for the group title Friday with the winner going against Vinci in Saturday's semifinals and the loser drawing Wozniacki. If Wozniacki can win that match, she will move to No. 10 in the world rankings.
The final Group Serdika match Friday pits Hsieh against Hantuchova.
Rosie O'Donnell unveils nearly 50-pound weight loss
Members of Congress to keep receiving porn magazine