Petrova on collision course with Wozniacki in Sofia
Nadia Petrova matched the exploits of Caroline Wozniacki and will take an unbeaten record into the knockout stages of the Tournament of Champions in Sofia.
The Russian defeated home favourite Tsvetana Pironkova 5-7 6-1 6-3 to win the four-player Sredets Group, although both players had already qualified for the semi-finals.
Petrova will face Italian Roberta Vinci in Saturday's semi-finals, while Pironkova meets top seed Wozniacki, winner of the Serdika Group at Armeets Arena.
Hsieh Su-Wei finished third in Wozniacki's group after she defeated Daniela Hantuchova 6-1 0-6 6-4 in a dead rubber.
Earlier on Friday, alternate Sofia Arvidsson, replacement for the sick Maria Kirilenko, was leading Zheng Jie when the sixth seed retired trailing 1-5 in the opening set.