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.
Reigning Australian Open champion 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, who was playing in her first US Open quarter-final, 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 on an overhead smash winner.
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 cannot wait for a mistake. You have to make your opportunities."
Azarenka avoided becoming the first US Open women's top seed to crash out before the semis since Dinara Safina's third-round exit in 2009.
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.
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. It was the first set Stosur had lost in the tournament.
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 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 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 tie-breaker while the tension mounted.
Azarenka leads Sharapova 5-4 in their career rivalry and leads Bartoli 8-3.
Sharapova, the 2006 US Open champion, would return to second in the world rankings next week if she defeats Bartoli as well as reach her first US Open final four since her championship run.
Wednesday's scheduled quarter-finals will send Williams, a 14-time Grand Slam champion, against Serbian 12th seed Ana Ivanovic and Italian 10th seed Sara Errani against 20th-seeded compatriot Roberta Vinci.