Serena Williams' US Open title defence ended in bizarre, ugly fashion, when she was docked 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 6-4, 7-5 in a taut semifinal that featured plenty of powerful groundstrokes by both women. No one will remember a single shot that was struck, though, because of the unusual, dramatic way it finished.

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 semifinals 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.

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.

"She was called for a foot fault, and a point later, she said something to a line umpire, and it was reported to the chair, and that resulted in a point penalty," Earley explained. "And it just happened that point penalty was match point. It was a code violation for unsportsmanlike conduct."

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.

"I used to have a real temper, and I've gotten a lot better," Williams said in her postmatch news conference. "So I know you don't believe me, but I used to be worse. Yes, yes, indeed."

Lost in the theatrics was Clijsters' significant accomplishment: In only her third tournament back after 2 1/2 years in retirement, the 26-year-old Belgian became the first mother to reach a Grand Slam final since Evonne Goolagong Cawley won Wimbledon 1980.

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