By Larry Fine

NEW YORK (Reuters) - Defending champion Kim Clijsters held off a gritty challenge from fifth-seeded Australian Samantha Stosur, surviving an error-filled three-set battle on Tuesday to reach the semi-finals of the U.S. Open.

With both players struggling to hold serve in tricky winds on Arthur Ashe center court, the second-seeded Belgian steadied herself at the end to complete a 6-4 5-7 6-3 win that set up a showdown against American Venus Williams.

"This whole 10 days has been really tough to get used to the weather conditions," said Clijsters, referencing the severe heat earlier in the tournament and the gusting winds of late.

"Every match is just a battle and mentally you just have to try and beat it."

Clijsters stopped a string of six successive service breaks at the start of the final set to seize a 4-3 lead before breaking Stosur again and then holding serve to end the match.

The victory was the 19th in a row for Clijsters at Flushing Meadows, including her championship run in 2005. Injury and then a temporary retirement to begin a family kept her away from the tournament until her triumphant return last year.

Williams, like Clijsters a two-time U.S. Open winner, was a straight-sets winner over French Open champion Francesca Schiavone in the day's other women's quarter-final.

French Open finalist Stosur, who saved four match points in defeating Russian Elena Dementieva in the fourth round, rose up again to force a third set, ending the last two games of the second set with forehand volleys to win her first set ever against Clijsters after three previous career defeats.

Stosur held serve to end the set after three successive service breaks in winds that made tosses difficult to manage. Clijsters made eight double faults in the 114-minute match.

The spate of service breaks to start the third set made for nine service breaks in a 10-game span before Clijsters righted the ship.

"I was just able to be a little more aggressive in that third set," the Belgian said. "Although my serve wasn't going as well as I would like it to go, I ended up winning."

Clijsters made 43 unforced errors and Stosur, who was up an early break in each of the sets, committed 36 in the mistake-filled, wind-tossed match.

(Editing by Frank Pingue)