NEW YORK:Serena Williams' US Open title defense ended in bizarre, ugly fashion on Saturday, when she was penalised a point on match point after yelling and shaking her racket in the direction of an official who called a foot fault.

Williams lost to unseeded, unranked Kim Clijsters of Belgium 6-4, 7-5 in a taut semi-final. The match featured plenty of powerful groundstrokes and lengthy exchanges. No one will remember a single shot that was struck, though, because of the unusual, dramatic way it ended.

With Williams serving at 5-6, 15-30 in the second set, she faulted on her first serve. On the second serve, a line judge called a foot fault, making it a double-fault -- a call rarely, if ever, seen at that stage of any match, let alone the semi-finals of a Grand Slam tournament.

That made the score 15-40, putting Clijsters one point from victory. Instead of stepping to the baseline to serve again, Williams went over and shouted and cursed at the line judge, pointing at her and shaking a ball at her. "If I could, I would take this... ball and shove it down your... throat," Williams said. She continued yelling at the line judge, and went back over, shaking her racket in the official's direction.

The line judge went over to the chair umpire, and tournament referee Brian Earley joined in the conversation. With the crowd booing -- making part of the dialogue inaudible -- Williams then went over and said to the line judge: "Sorry, but there are a lot of people who've said way worse." Then the line judge said something to the chair umpire, and Williams responded, "I didn't say I would kill you. Are you serious? I didn't say that." The line judge replied by shaking her head and saying, "Yes."

Williams already had been give a code violation warning when she broke her racket after losing the first set. So the chair umpire now awarded a penalty point to Clijsters, ending the match. When the ruling was announced, Williams walked around the net to the other end of the court to shake hands with a stunned Clijsters, who did not appear to understand what had happened.

Lost in the theatrics was Clijsters' significant accomplishment: In only her third tournament back after two and a half years in retirement, the 26-year-old Belgian became the first mother to reach a Grand Slam final since Australia's Evonne Goolagong Cawley won Wimbledon 1980.

Clijsters hadn't competed at the US Open since winning in 2005. Now she will play for her second career major title against No 9 Caroline Wozniacki of Denmark, who beat Belgium's Yanina Wickmayer 6-3, 6-3 in the other rain-delayed semi-final.

Williams came into the day having won three of the past four Grand Slam titles, and 30 of her previous 31 matches at major tournaments. She was playing fantastically at the US Open, not losing a set before Saturday and having lost her serve a total of three times through five matches.