Wozniacki, who will be the top seed at the U.S. Open next week, was late arriving in New Haven after playing a semi-final and final at the rain-hit event in Montreal on Monday and appeared out to struggle in the opening set.
With Dementieva serving to stay in the match at 6-5, Wozniacki had triple break point only to watch the Russian sweep the next five points to force a tiebreak, which she recovered to win 7-5 on her first match point.
The eighth seeded Russian rallied for a 2-6 6-2 6-2 win over compatriot Maria Kirilenko, who was bothered by a sore back that required treatment during the match.
Petrova dropped her serve twice in the opening set but would not be broken again reaching her 20th career final.
"When Maria called for the physio, those couple of minutes did very well for me," Petrova, told the WTA's official website (www.sonyericssonwtatour.com.). "I saw she was struggling and believed I could come back and win.
"I cut down on my mistakes and focused a lot on my serve."
(Writing by Steve Keating in Toronto; Editing by John O'Brien)