In a match between two unseeded players, Petrova got early breaks in both sets against the 19-year-old Azarenka, who had upset Olympic champion Elena Dementieva in the quarterfinals.
"I am so excited to be back in the final,'' said Petrova, a Russian who won the tournament in 2006. "The main thing for me is to stay focused, then everything goes well for me.''
Petrova is looking for her second title of the year after winning in Cincinnati. Two years ago, Petrova was ranked third in the world but she has now dropped to No. 18.
She jumped to a 4-0 lead before Azarenka won the next three games. But Petrova stayed ahead to win the first set. Petrova also led 2-0 in the second, then lost the next three games but broke serve for a decisive 4-3 lead and won with a service winner on her second match point.
Petrova will play either Jelena Jankovic or Venus Williams in Sunday's final.
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