World No. 1 Victoria Azarenka advanced to the US Open semi-finals for the first time on Tuesday with a 6-1, 4-6, 7-6 (7/5) victory over seventh-seeded defending champion Samantha Stosur of Australia.

The 23-year-old top seed from Belarus ensured that she will remain atop the world rankings no matter what happens the rest of the way in the year’s final Grand Slam tournament on the Flushing Meadows hardcourts.

“I don’t want to stop yet,” Azarenka said.

Azarenka advanced to a semi-final against either French 11th seed Marion Bartoli or Russian third seed Maria Sharapova, who completed a career Grand Slam by winning this year’s French Open.

Azarenka weathered a 75-minute rain delay in the first set to win for the sixth time in as many top-level matches against Stosur.

“She played amazing. The level was so much harder today,” Azarenka said. “She made me play my best.”

In the final set tie-breaker, Azarenka raced to a 4-0 lead after Stosur sent a forehand beyond the baseline but the Aussie battled back to level at 5-5.

Azarenka seized a 6-5 lead on a drop volley after Stosur’s shot struck the net cord and plopped over and won the match when the Aussie sent a backhand wide after two hours and 23 minutes.

“We both showed some excellent tennis,” Azarenka said. “You could feel the pressure all the time. We really both gave it all. We fought really hard, making each other run like crazy.”

It was the first US Open women’s match in the last-eight or deeper to go to a third-set tie-breaker since 2007, Azarenka saying it was a greater mental challenge than physical one at that point.

“Physically we were already giving all on every point,” she said. “You accept to take that risk or take that challenge. You have to make your opportunities.”

Azarenka broke Stosur in the first and third games of the opening set before showers struck Arthur Ashe Stadium and halted the match after 11 minutes.

“It’s never pleasant to stop the momentum but it was the same for both of us,” Azarenka said.

When play resumed, Azarenka added a break to close out the set, which she claimed after 30 minutes when Stosur sent a backhand beyond the baseline.

Stosur broke to open the second set. Azarenka broke back in the next game but Stosur broke again to 4-3 with a forehand down-the-line winner and held twice to force a third set. It was the first set Azarenka had dropped in the tournament.

Stosur surrendered a break in the fourth game of the third set by netting a backhand, broke back in the next game , then handed back the break by sending a backhand long to give Azarenka a 4-2 edge.

But Stosur answered again, firing forehand winners to break back as both then held serve into the tie-breaker while the tension mounted.

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