NEW YORK — Serena Williams’ U. S. Open title defense ended in bizarre, ugly fashion Saturday night, when she was penalized 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.

“If I could, I would take this . . . ball and shove it down your . . . throat and kill you,” Williams said.

The line judge went over to the chair umpire, and tournament referee Brian Earley joined in the conversation. Williams then went over and said to the line judge: “I didn’t say I would kill you. Are you serious? Are you serious? I didn’t say that.”

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 in1980.

Clijsters hadn’t competed at the U. S. Open since winning the 2005 championship. Now she will play for her second career major title tonight (9 p. m., ESPN2) 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.

On the men’s side, third-seeded Rafael Nadal beat No. 11 Fernando Gonzalez, 7-6 (4), 7-6 (2), 6-0, in a match that began Thursday evening, was suspended that night because of showers in the second-set tiebreaker, and didn’t resume until Saturday thanks to more rain Friday.

Weather permitting — two key words at Flushing Meadows lately — Nadal will face No. 6 Juan Martin del Potro in one men’s semifinal today (Noon, ESPN2), and No. 1 Roger Federer will meet No. 4 Novak Djokovic in the other (4 p. m., Ch. 4).

Gonzalez, for one, figures this year’s championship matchup is a foregone conclusion.

“You always expect that Federer plays the finals against Nadal,” Gonzalez said. “Good for the game. It’s good for the fans.”