Two-time champions Kim Clijsters and Venus Williams booked a US Open semifinal showdown with hard-fought victories on Tuesday, struggling in windy conditions to overcome their toughest foes yet.

Defending champion Clijsters, trying to become the first winner to repeat since Williams in 2001, struggled but stretched her Flushing Meadows win streak to 19 matches by ousting Australian fifth seed Samantha Stosur 6-4,5-7,6-3.

"I didn't play a good match but I was able to win it," Clijsters said. "Even after the match I waslike, 'How didi win this?' Next match everything will have to be a lot better."

US third seed Williams, a seven-time Grand Slam champion who has not won a US Open crown since completing her back-to-back feat, eliminated sixth-seeded French Open winner Francesca Schiavone of Italy 7-6(7/5), 6-4.

"I was trying to find a rhythm in the wind to getontopofit," Williams said." It was challenging. I don't think either of us were able to play our exact normal game. When the stakes were higher I was able to raise my game."

Entering Friday's semifinal, Clijsters and Williams have split 12 matches, although Williams has lost their past four meetings, including a 2005 US Open quarterfinal and a fourth-round match last year that went three sets.

"We had a great match last year. It was really close," Williams said. "I'm sure we'll have another really good matchup. I would like to kind of flip the way it turns out."

Williams, who has yet to drop a set in this Flushing Meadows fortnight, became the first 30-year-old woman to reach a Grand Slam semifinal since France's Mary Pierce at the 2005 US Open.

"She has been playing really well,” Clijsters said. "It's a challenge. I look forward to it."

Clijsters and Stosur exchanged breaks in the first six games of the third set before the Belgian held serve, then broke Stosur again when the Aussie double faulted for a 5-3 edge. Clijsters then held again for the victory.

"I just had to be aggressive in the third set and keep going when my serve wasn't as good as I would like it," Clijsters said.

French Open runner-up Stosur, now 0-4 against Clijsters, missed a chance to become the first Aussie woman in the US Open semifinals since Wendy Turnbull in 1984.

"I'm going to be disappointed about this one," she said. "You can't break serve and have chance after chance after chance and blow it in a quarterfinal. It's just too big an opportunity to let go. Unfortunately that's what I did."

Stosur could take some consolation in her best-ever US Open run.

"I'm very pleased," she said. "It's just unfortunate it had to end like this."

"It wasn't easy out there to find your best tennis. Windiest conditions I've played in all week and probably the worst I've served all week. Put those two things together and don't hold serve for the third set and you lose."

Schiavone, who fell to 0-8 against Williams, was similarly frustrated after her defeat.

"I think I lost a little bit more this match than she won," Schiavone said. "It's a lot of disappointment. I have the quality and the chance to beat her."

Schiavone and Williams exchanged breaks to open the match and each double faulted to surrender another break on the way to a first-set tie-breaker.

Williams led 4-0, Schiavone levelled at 4-4 but the Italian swatted a pair of forehands beyond the baseline on the last two points to hand Williams the set.

In the second set, Williams broke Schiavone for a 3-1 lead and the rivals traded service breaks over the last four games to finish the match.