Kim Clijsters admitted she was surprised to have reached the US Open semi-finals soon after coming out of her retirement.

Playing on a wild card in just her third tournament since coming out of retirement having become a mother for the first time, Clijsters breezed past China's 18th seed Li Na in just 63 minutes, winning 6-2 6-4 to set up a mouth-watering last-four clash with defending champion Serena Williams.

The victory was the 26-year-old Belgian's 100th career grand slam win in her first US Open since winning it in 2005.

Clijsters, who had beaten third seed Venus Williams in three sets in the previous round, also secured her sixth straight semi-final appearance in a grand slam having retired in May 2007 following an Australian Open last-four loss to Maria Sharapova the preceding January.

"I never had the mentality before the tournament that I would get to a semi-final or final at the grand slams," Clijsters said.

"I definitely didn't have that idea but something switched with me after Toronto (her second tournament back) where I felt like 'Okay, I can compete with these girls', because that was obviously a big question in my mind.

"Although I was hitting the balls really good in practice and I felt physically I was doing well you still want to know what it's like to play against those girls and get a feel for how the level has changed.

"That was something I didn't know, so I am surprised that I'm sitting here right now but I'm very happy."

It was a straightforward victory for Clijsters as she prevented Li from becoming the first Chinese player to reach a grand slam semi-final. The Belgian went a break up in the third game and did not look back as she wrapped up the opening set in 25 minutes.

It took nine games for Clijsters to make the breakthrough in the second set but once she got it the wild card wasted no time in sealing victory, earning double match point in the next game.

Clijsters netted a forehand on the first point but victory was hers when Li returned a second serve on the next one with a wild forehand.

"She's so much stronger than other girls," Li said. "Maybe it's a day for her, not for me."

Williams beat Italy's Flavia Pennetta 6-4 6-3 in their quarter-final tonight.

The defending champion's serve played a significant part in her victory as she hit seven aces and finished with an 86% first serve success.

The American is now relishing a showdown with the returning Clijsters.

Williams said: "She is such a great person and I only wish the best for her - though not in the next match of course.

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

"Initially I was surprised how well she's been doing but she did so well a couple of weeks ago that I knew she was someone to look out for."