CINCINNATI, Ohio (AFP) - Victoria Azarenka toppled world No. 1 Serena Williams 2-6, 6-2, 7-6 (8/6) to claim the Cincinnati WTA title and establish herself as a contender for the US Open.

The two-and-a-half-hour epic featured multiple momentum shifts after Azarenka, the second seed from Belarus, began the afternoon with two double faults to lose the opening set in under 30 minutes.

She managed to steady herself and held on to beat the top-seeded Williams for only the third time in 15 matches. It was her second victory over the American this year, after a triumph in the final at Doha.

"What doesn't kill you makes you stronger," said Azarenka who earned her 17th career title and first in Cincinnati. "It was a tough match and we both fought really hard. It was an incredible week for me."

Azarenka had played sporadically over the past months due to injuries suffered at both Wimbledon and this month in California. The Belarussian was competing in only her tenth event of an injury-hit season.

"I'm really excited to be back playing," said the former world number one.

"The last month was really tough." Azarenka lost a second set in which Williams saved eight break points in the sixth game. With the sets level, the third went to a decider as the pair traded four breaks in five games.

Williams double-faulted for a match point in the tiebreaker, but Azarenka gave it back with an error. Azarenka then shipped over a winner for a second winning chance and followed through for the win a point later.

Williams had been trying for two major hardcourt titles in a row after winning Montreal last weekend.

Between them, Williams and Azarenka have claimed five of the last seven Grand Slam singles titles dating to the 2012 Australian Open.

Williams beat her rival in the US Open final a year ago.