The confidence-boosting 7-5 7-6(5) victory sent Keys into the US Open later this month as the world's No.10 ranked player, officials said.
"If you told me a week ago this is where I would be, I would have laughed in your face!" Keys said in accepting the tournament trophy.
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In both sets, the 2017 US Open finalist trailed 5-3 before rallying to win.
Keys won four consecutive games to claim the opener. She needed a tiebreak to prevail in the second.
The triumph was the fourth in a row on hardcourts for the 16th-seeded Keys over Kuznetsova.
Keys delivered 13 aces and 45 winners against 29 unforced errors. Kuznetsova had just two aces and 15 winners.
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