NEW YORK: Defending champion Serena Williams will play comeback queen Kim Clijsters for a place in the US Open final after both scored straight sets wins in their quarter-finals.

Williams, seeded second despite being widely regarded as the best player in the world, defeated Italy's Flavia Pennetta 6-4, 6-3.

Clijsters, bidding to become just the third mother to win a Grand Slam title in the modern era, saw off the challenge of China's Li Na 6-2, 6-4. The Belgian had defeated Serena's sister Venus in the fourth round.

The win over Pennetta was a fifth consecutive straight-sets win for Williams, who has been in imperious form as she seeks a fourth US Open crown and a 12th Grand Slam title overall.

The 27-year-old Italian, who has belatedly stepped out of the shadows this year to become the first woman from her country to break into the world top 10, gave as good as she got in the early exchanges against Williams.

She got to 4-4 before Williams started to turn on the power as and when she needed, holding serve easily and then break Pennetta's serve to love.

The Italian did have two break points in the opening game of the second set but both times under pressure she netted a forehand.

The 10th seed, playing in just her second Grand Slam quarter-final, once again stood her ground valiantly until the sixth game when again Williams stepped on the turbo to force a break point on which Pennetta double-faulted.

That was all the American needed to set up the clash with Clijsters.

"She is such a great person and I only wish the best for her," Williams said of the Belgian, who only returned to tennis in early August after taking more than two years out to marry and have a baby.

"It will be a great match. She has nothing to lose and is having so much fun. It makes me so happy.

"Intitially I was surprised (how well Clijsters was doing on her comeback), but she did so well a couple of weeks ago that I knew she was someone to look out for."

Clijsters, who was given a wildcard to be able to compete at Flushing Meadows, dominated her match against Li apart from a spell in the second set when the Chinese No.1 won four games in a row to lead 4-3.

But the former world No.1 who won her only Grand Slam title here in 2005, promptly reasserted control to rattle off the next three games for the win.

"Obviously I feel good about it. It's a nice feeling to have, knowing I was able to win it in two sets," Clijsters said.

"Even in that second set I got a little bit closer, but I was just able to finish it off in straight sets."

Turning to her semi-final opponent, Clijsters said: "Serena is a great athlete and a great girl and we always have some good laughs in the locker rooms and that's what makes it fun as well."

The win and semi-final place meant Clijsters equalled the best recorded run by a wildcard, man or woman, at the US Open, matching Jimmy Connors in 1991.

The defeat was a heavy blow for Li, who was bidding to become just the second Chinese player ever to reach a Grand Slam semi-final, matching Zheng Jie's wildcard run into the Wimbledon last four in 2008.

It was Clijsters who also stopped Li the last time she reached a Grand Slam quarter-final at Wimbledon 2006.

The two remaining quarter-finals will be played on Wednesday in a half of the draw where only one seed made it through to that stage.

In a clash of two unseeded players, Yanina Wickmayer of Belgium will play Kateryna Bondarenko of Ukraine, while 17-year-old American sensation Melaine Oudin will take on Danish ninth seed Caroline Wozniacki.