Victoria Azarenka survived a ferocious comeback from defending champion Sam Stosur to reach the U.S. Open semi-finals on Tuesday, as the remnants of Hurricane Isaac played havoc with the schedule and left organizers juggling with a backlog.

Only two of six singles matches were completed due to rain, forcing tournament officials to reschedule the remaining four for Wednesday in the hope they can finish the year's last grand slam on time.

However, the prospects are bleak with more showers forecast in the 'Big Apple' for the rest of the week.

Azarenka traded blows with Stosur for nearly two and a half hours on a blustery Arthur Ashe Stadium court to beat the foul weather and her opponent in a 6-1 4-6 7-6 thriller.

The world number one will play either Maria Sharapova or Marion Bartoli in Friday's semi-finals and will remain atop the world rankings regardless of how much further she goes in the tournament.

"It means a lot, but it's nothing like lifting a trophy," said Azarenka. "I definitely don't want to stop. I really want it bad."

While the defeat was disappointing for Stosur, the big-hitting Australian was satisfied to have proved that her surprise win here last year was no fluke.

"That proves to me that I am capable of doing it," she said. "To have another showing here at the Open like this, it for sure gives me confidence to think that maybe one day I can do it again."

The only other match that went the distance saw Spain's David Ferrer defeat Richard Gasquet ofFrance 7-5 7-6 6-4 in the fourth round.

The match took almost eight hours to complete as the heavens opened up and drenched Flushing Meadows. Three other men's fourth round matches were suspended in the first set.

Defending men's champion Novak Djokovic was leading Stanislas Wawrinka 2-0, while 2003 winner Andy Roddick and Juan Martin Del Potro, the 2009 U.S. Open champion, had played one point of their tibreaker. Janko Tipsarevic was 5-2 up against Philipp Kohlschreiber.

In the women's quarter-finals, Bartoli grabbed an early 4-0 lead against Sharapova, one of six former U.S. Open champions still in contention.

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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 to be cruising to another easy win when she romped through the opening set in just 30 minutes.

But Stosur, who upset Serena Williams in last year's final, found her rhythm after an initial 75-minute delay, landing more of her booming first serves and hitting her 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.

"I enjoy the fight," said Azarenka. "I enjoy the struggle, that pain that we go through, that incredible moment that you feel relieved after you know you gave it all in every point you had."

Azarenka's opponent in the last four has yet to be decided after rain played havoc with the day's schedule.

At least 30 people were killed by Hurricane Isaac during its deadly trek across the Caribbean, Louisiana and Mississippi and while the storm has weakened, rain and flash floods have been forecast for the U.S. East Coast where the U.S. Open is played.

In each of the last four years, the tournament has stretched into a third week because of thunderstorms, triggering an annual debate and complaints from players and spectators over why there is still no roof over the courts.

The centre courts at both the Australian Open and Wimbledon have retractable roofs while organizers of the French Open plan to cover up their main court by 2014.

However, the United States Tennis Association has balked at the idea because of the enormous cost of covering Arthur Ashe Stadium, the largest tennis stadium in the world.