Samantha Stosur reacts after winning the women's championship match against Serena Williams at the U.S. Open in New York, Sunday. Stosur won, 6-2, 6-3.

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.”

Hitting powerful strokes from the baseline, and looking fresher than the far-more-accomplished Williams right from the start, the ninth-seeded Stosur became the first Australian woman to win a major championship since Evonne Goolagong Cawley at Wimbledon in 1980.

Only 2-9 in tournament finals before beating Williams, Stosur made the U.S. Open the third consecutive Grand Slam tournament with a first-time women’s major champion, after Li Na at the French Open, and Petra Kvitova at Wimbledon.

“She played really, really well. I mean, I don’t think she’s ever played that well,” Williams said. “Maybe she has; I haven’t seen her play that well.”

This was only the 27-year-old Stosur’s third title at any tour-level event, and what a way to do it. She took advantage of Williams’ so-so serving and stayed steady throughout — finishing with 12 unforced errors to Williams’ 25 — despite the bizarre events that unfolded in the second set.

Indeed, the biggest victory of Stosur’s career so far likely will be recalled by everyone else for Williams’ latest dispute with an official at Flushing Meadows.

Down a set and facing a break point in the first game of the second, the 13-time major champion hit a forehand and shouted, “Come on!” as Stosur reached down for a backhand. Chair umpire Eva Asderaki ruled that Williams hindered Stosur’s ability to complete the point and awarded it to Stosur — putting her ahead 1-0 in that set.

Williams went over to talk to Asderaki, saying, “I’m not giving her that game.”

Williams also said: “I promise you, that’s not cool. That’s totally not cool.”

Some fans began booing, delaying the start of the next game as both players waited for the commotion to subside.

Tournament director Brian Earley said Asderaki’s ruling was proper, according to U.S. Tennis Association spokesman Chris Widmaier.

International Tennis Federation rules say: “If a player is hindered in playing the point by a deliberate act of the opponent(s), the player shall win the point. However, the point shall be replayed if a player is hindered in playing the point by either an unintentional act of the opponent(s), or something outside the player’s own control (not including a permanent fixture).”

Williams said later she thought that last part of the rule applied — and the point should have been replayed — such as when one player’s hat flies off during a point.

In the heat of the moment, Williams had trouble putting the whole episode behind her.

During the changeover two games later, Williams continued to talk to Asderaki, saying, “You’re out of control. … You’re a hater, and you’re just unattractive inside.”

Williams also told the official: “Really, don’t even look at me.”

At her news conference, Williams rolled her eyes while deflecting a question about whether she regretted what she said to Asderaki.

“I don’t even remember what I said. It was just so intense out there. … I guess I’ll see it on YouTube,” Williams said.

When Stosur wrapped up the match with a forehand winner, Williams refused the customary post-match handshake with the chair umpire.

“I hit a winner, but I guess it didn’t count,” Williams said during the trophy presentation. “It wouldn’t have mattered in the end. Sam played really well.”