Petrova also secured the second alternate berth for next week's season-ending WTA Tour Championships in Doha. The Russian has played in the elite event twice previously.
Petrova advances to play American Angela Haynes, who lost in qualifying but made the draw when Flavia Pennetta withdrew. Fifth-seeded local Aleksandra Wozniak will meet No. 6-seed American Bethanie Mattek in the other semifinal.
Wozniak, who started off the week receiving a medal from the state parliament for becoming the first woman from Quebec to rank in the world's top 50, defeated Galina Voskoboeva of Kazakhstan 6-2, 6-2. The Montreal-born player moved into the Quebec City semifinals for the first time.
Mattek ended the fine run of 17-year-old countrywoman Melanie Oudin 7-6 (7), 6-1, and Haynes ousted French veteran Nathalie Dechy 2-6, 7-6 (2), 6-3 to reach her first WTA Tour semifinals.
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