Belgian comeback queen Kim Clijsters knocked out Serena Williams in a wildly controversial finish on Saturday to advance to the final of the US Open and become the poster girl for working mothers.

Just weeks after returning to the tour from a two-year break to start a family, Clijsters beat the defending champion 6-4, 7-5 after a day-long rain delay at Flushing Meadows.

At 5-6, 15-30 down in the second set, Williams whacked her second serve but the lineswoman called her on a foot-fault that put her at match point down.

The American's subsequent expletive-laced tirade directed at the lineswoman resulted in a point penalty -- and the end of the match.

"I swear to God I'm... going to take this... ball and shove it down your... throat, you hear that? I swear to God," Williams said.

After the line-judge reported the second seed to the umpire for verbal abuse, Williams added: "I never said I would kill you, are you serious?"

In Sunday's final, Clijsters will face Danish teenager Caroline Wozniacki, who defeated unseeded and error-prone Belgian Yanina Wickmayer 6-3, 6-3 in the other semi-final.

Earlier on Saturday, third seed Rafael Nadal needed just 34 minutes to complete a rain-delayed 7-6, 7-6, 6-0 victory over Chile's Fernando Gonzalez and gain a spot in the semi-finals.

Nadal will face sixth seed Argentine Juan Martin del Potro, one of the hottest players on the tour this summer.

The contentious ending of Clijsters's match marred her well-earned victory.

"It's unfortunate that a match that I was playing so well at had to end that way," said Clijsters, the 2005 Open champion and former world number one.

"Obviously, I still to this point am a little confused about what happened out there, just because I was so focused. I was trying to win that last point. Things ended a little bit different than I expected."

Clijsters, the first mother to reach a grand slam final since Evonne Goolagong at Wimbledon in 1980, missed out on the thrill of winning match point.

"When you play that last point, whether it is a winner or by mistake from your opponent, it's a great feeling to have," Clijsters told reporters.

"So, yeah, the normal feelings of winning a match weren't quite there."

Williams, the 2009 Wimbledon and Australian Open champion, could not believe her misfortune.

"All year I've never been foot faulted, and then suddenly in this tournament they keep calling foot faults," Williams said.

"I said something that I guess they gave me a point penalty for. Unfortunately it was on match point."

In a match delayed more than seven hours because of wet conditions, unseeded Wickmayer made 40 unforced errors, mainly from her booming forehand, against just 14 for Wozniacki.

"I'm in the US Open final, I cannot describe it with words," said Wozniacki, the first Danish woman to reach a grand slam singles final. "I'm so excited. It's a dream come true.

"I have absolutely nothing to lose."