New York, Sep 5 (IANS): World No.1 Victoria Azarenka advanced to the US Open semifinals for the first time with a 6-1, 4-6, 7-6(5) victory over seventh-seeded defending champion Samantha Stosur of Australia here at the Flushing Meadows.

The 23-year-old top seed from Belarus ensured Tuesday that she will remain atop the world rankings no matter what happens hereon in the year's final Grand Slam, reports Xinhua.

The reigning Australian Open champion will next face either French 11th seed Marion Bartoli or Russian third seed Maria Sharapova.

The match between Bartoli and Sharapova was postponed to Wednesday after rains disrupted play when Bartoli led 4-0 against the Russian.

In the final set tiebreaker, 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 thanks to an overhead smash winner and a double fault from Azarenka.

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.

"I enjoy the fight," Azarenka said.

"I enjoy that struggle, that pain that we go through, that incredible moment that you feel relieved after you gave it all in every point you had. I don't want to stop yet."

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

After 10 minutes of rain, Azarenka won the first set 6-1. This was the first set Stosur lost in the US Open.

Stosur, seeking her first title since lifting the trophy last year on the same court, 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 serve twice to force a third set. This became 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. She broke back in the next game on a forehand service return winner, then handed back the break by sending a backhand long to give Azarenka a 4-2 edge.

But Stosur answered again firing back-to-back forehand winners to break back as both then held serve into the tiebreaker.

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.

But no women's top seed has won a Grand Slam title since Serena Williams at Wimbledon in 2010 and no US Open women's top seed has taken the title since Justine Henin in 2007.