Azarenka, who had to go through Sloane Stephens and No.8 seed Caroline Wozniacki en route to the third round, looked headed for a routine straight set victory as she built a 6-4, 3-0 lead, her big ground strokes hitting their mark as Zahlavova Strycova kept searching for the right answers.

She found them for a while - the No.25-seeded Czech caught up to 3-all, then after losing serve in the next game broke back for 4-all as well. But Azarenka held her nerve too well, breaking one last time and serving it out, wrapping up a gruelling 6-4, 6-4 victory after an hour and 41 minutes on court.

Azarenka has now won 21 of her last 22 matches here, winning back-to-back titles in 2012 and 2013, reaching the quarter-finals in 2014 and now three more wins in 2015.

Azarenka was asked afterwards whether she's feeling more confident with each win here.

"I take it as progress. It was another opportunity to improve. I had a different opponent, so it's difficult to compare the level or the quality because it's different styles, but I'm just happy I managed to go through my matches," Azarenka said.

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