Victoria Azarenka toppled world number one Serena Williams 2-6 6-2 7-6 (8/6) on Sunday to win the Cincinnati WTA title and signal her US Open intentions.

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.

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