NEW YORK, NY, USA - Emotions were high and there was drama aplenty in New York on Saturday night, as Serena Williams' temper got the better of her and cost her a chance to defend her US Open title.

Williams, the No.2 seed and defending champion, took on former No.1 Kim Clijsters, playing just the third tournament of her 2009 comeback, and a wildcard into the draw. Williams was sharp but Clijsters was sharper, hanging with the 11-time Grand Slam champion from the back of the court but also keeping her numbers cleaner. While Clijsters erred seven times in the opening set, Williams had 14, including a backhand into the net on set point, which prompted her to smash her racquet on the ground, giving her a warning.

The competitive fires kept burning in the second set, with the players trading huge groundstroke for huge groundstroke and running down just about everything humanly possible, until that one fateful foot fault moment.

Serving at 5-6, 15-30, Williams was called for a foot fault on a second serve, giving Clijsters double match point, 15-40. But the tirade that Williams unleashed on the baseline judge would cost her the match - she had already acquired a warning earlier on, now a point penalty - at the worst time.

"I said something and they gave me a point penalty. Unfortunately, it was on match point," Williams said. "You know, today was a tough day. I didn't play my best. I think I had more errors today than all of my other matches combined.

"I think Kim played really well, and I think she came out with a really big plan. I saw her play in Cincinnati and she played incredible. It's really good to have her back on tour. Maybe we can get together and have some calming lessons."

And what about the foot fault? "I'm pretty sure I did," Williams added. "If she called a foot fault, she must have seen a foot fault. I mean, she was doing her job. I'm not going to knock her for doing her job."

Clijsters, who is projected to rise to around No.30 should she fall in the final and into the Top 20 with a title, is trying to become the first wildcard in Sony Ericsson WTA Tour history to win a Grand Slam singles title. Apart from the feelings regarding the end to the match, Clijsters was happy to win.

"It's unfortunate that a match I was playing so well in had to end that way," Clijsters said. "I'm still a little confused about what happened, because I was so focused. The normal feelings of winning a match weren't quite there, but when everything sunk in a little bit and what happened got explained to me, it became easier to understand, not to celebrate, but at least to have a little joy."

Facing Clijsters in the final will be No.9 seed Caroline Wozniacki, who beat surprise semifinalist Yanina Wickmayer, 63 63. Wozniacki kept her game steady and nerves in check while Wickmayer's game was all over the place, making 40 unforced errors. Wozniacki had just 14 the whole match.

"I'm in a Grand Slam final. I'm in the US Open final. I cannot describe it with words. I'm so excited," said Wozniacki, who had never even been to the quarters of a major before this tournament. "It's a dream come true to play the finals of a Grand Slam, and now I'm here. I also have absolutely nothing to lose."

Clijsters and Wozniacki have never played against each other, but they have been on the same court together.

"Kim and I played an exhibition together in Hong Kong, I think it was in 2006. We played the doubles together," Wozniacki said. "She's such a sweet girl. She's such a nice person. I'm so happy she's back on tour."

Clijsters, who beat Venus Williams in the round of 16, is the first player ever to beat both Williams sisters at the same tournament twice, having also done it at the Sony Ericsson Championships in 2002 (semifinals and final).