Petrova, the No. 2 seed in the tournament, beat eighth-seeded Tsvetana Pironkova 5-7, 6-1, 6-3 in winning Group Sredets.
While Thursday's results had determined Saturday's tournament semifinalists, the matchups weren't determined until the Petrova-Pironkova match was over. As a result Petrova will take on Group Serdika runner-up Roberta Vinci and Pironkova goes against top-seeded Caroline Wozniacki, who was 3-0 in her group.
Petrova lost her serve just once over the second and third sets, winning about two-thirds of the points on serve over those sets in the win
Also Friday, Sofia Arvidsson, filling in in Group Sredets for Maria Kirilenko who withdrew due to illness, came out ahead of Zheng Jie 5-1 when Zheng retired.
Hsieh Su-Wei claimed the final round-robin match, beating Daniela Hantuchova 6-1, 0-6, 6-4. Hsieh was 1-2 in Group Serdika and Hantuchova went winless.
Wozniacki, 3-0 lifetime versus Pironkova, needs a win Saturday to finish the year in the Top 10 in the rankings. Petrova and Vinci have split their four previous meetings.
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