NEW YORK (Reuters) - Belarusian Victoria Azarenka survived a ferocious comeback from defending champion Sam Stosur on Tuesday to reach the U.S. Open semi-finals and ensure she remains world number one following the year's last grand slam.

Victoria Azarenka of Belarus celebrates after defeating Samantha Stosur of Australia in their women's singles quarterfinals match at the U.S. Open tennis tournament in New York September 4, 2012. REUTERS/Eduardo Munoz

The pair, two of the biggest hitters in women's tennis, traded blows for nearly two and a half hours in a match of unrelenting tension on a blustery Arthur Ashe Stadium centre court before Azarenka triumphed 6-1 4-6 7-6.

"I have no words," said a relieved Azarenka. "I think we both showed some excellent tennis.

"We both felt the pressure sometimes, we both gave it our all and we made each other run like crazy."

Azarenka, who won her first grand slam title at this year's Australian Open, had won her previous six matches against Stosur in straight sets and seemed headed to another easy win when she won the first four games and took the rain-interrupted opening set in just 30 minutes.

But Australian Stosur, who upset Serena Williams in last year's final at Flushing Meadows, found her rhythm after the 75-minute interruption, landing more of her booming first serves and hitting groundstrokes deeper and with greater conviction.

The seventh-seed won the second set then recovered from a break down in the third to draw level and force a deciding tiebreak, which Azarenka won 7-5 after blowing a 4-0 lead.

Azarenka will play either Russia's Maria Sharapova or France's Marion Bartoli in Friday's semi-finals.

(Editing by Frank Pingue)

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