NEW YORK — Already being outplayed by Sam Stosur in the U.S. Open final, the last thing Serena Williams needed was to lose a game for yelling during a point.

That’s exactly what happened early in the second set, leading to an argument between Williams and the chair umpire, a scene less ugly than — but reminiscent of — the American’s tirade two years ago at the same tournament. In the end, Stosur beat Williams 6-2, 6-3 Sunday in a surprisingly lopsided upset for her first Grand Slam title.

“I think I had one of my best days,” Stosur said. “I’m very fortunate to do it on this stage.”

“She played really, really well. She’s a great player, and it’s good to see,” Williams said. “I tried my hardest and she kept hitting winners and I was, ‘Oh my God, what am I doing?’