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What a game it was and what an opportunity for Victoria Azarenka to announce herself as one of the contenders for the US Open.
Azarenka on Saturday, dumped out 11th seed Angelique Kerber in one of three results to go against seeding.
Azarenka, seeded 20th, wrestled her way to a 7-5 2-6 6-4 victory in just under three hours on Arthur Ashe Arena, the two-time finalist in New York now forging herself as one of the main threats to Serena Williams' Grand Slam bid.
Spectators at Flushing Meadows' centre court could hardly complain about the quality of tennis on show, with Azarenka muscling 51 winners - and Kerber 46 - in an enthralling battle.
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Kerber served for the opening set at 5-3, but would end up losing four service games in the first set as Azarenka saved a set point to lead.
The German lefty was much more assured on serve in the second, not even giving Victoria Azarenka a whiff of a break point as she squared the match with two breaks.
The pair exchanged breaks early in the third, but Azarenka's breakthrough in the fifth game would be decisive - not before Kerber saved five match points in the ninth game to force the Belarusian to serve for victory.
Azarenka's result may have been an upset according to the seedings, but she remained unbeaten against Kerber in five meetings.
Other upsets saw unseeded Briton Johanna Konta dump out 18th seed Andrea Petkovic 7-6 (7-2) 6-3, while former champion Sam Stosur - the 22nd seed - ousted 16th seed Sara Errani 7-5 2-6 6-1.
There were simpler wins for fifth seed Petra Kvitova and second seed Simona Halep, untroubled on their respective paths to the last-16.
Kvitova got past 32nd seed Anna Schmiedlova 6-2 6-1, while Halep downed Shelby Rogers 6-2 6-3.
Azarenka's next challenge is unseeded American Varvara Lepchenko, who overcame a slow start to beat Mona Barthel 1-6 6-3 6-4.