Serena Williams approaches the unidentified lineswoman after the contentious call in Saturday's match. (Darron Cummings/Associated Press)

Serena Williams has been handed a $10,000 US fine for the profane outburst that led to her defeat late Saturday night at the U.S. Open.

It's the maximum on-site fine a Grand Slam tennis tournament can issue for unsportsmanlike conduct, but it may only be the beginning.

Open officials said Sunday the Grand Slam committee administrator has opened an investigation to determine whether Williams's behaviour should be considered a "major offence," which can lead to additional penalties.

"The average individual would look at that and say, 'A $10,000 fine for what she did? What are you guys, crazy?' The answer is: the process isn't over," tournament director Jim Curley said in an interview with The Associated Press.

Williams, 27, is also being slapped with a $500 US fine for racket abuse in what became a 6-4, 7-5 loss Saturday night.

Tournament spokesman Chris Widmaier said earlier on Sunday that officials were watching tapes of what happened Saturday night. The tournament referee, Brian Earley, interviewed Williams and the match’s chair umpire before they left the grounds Saturday.

Williams could have been ruled ineligible to play in the women's doubles final on Monday, although that would in essence also punish her older sister Venus.

The United States Tennis Association declined to name the lineswoman who riled Williams.

Williams boils over

Things boiled over in the match with Williams serving at 15-30 in what ultimately was the final game. The two-time defending champion was three points from sending the set to a tiebreak, but also two points away from defeat.

On her second serve, the lineswoman cited Williams for a foot fault to put her point away from defeat, a call that drew gasps from the crowd.

After a few seconds, Williams walked towards the lineswoman and pointed her racket.

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

Because she had already been cited for a code violation for throwing her racket in the first set, the second violation resulted in the loss of a point, giving the match to Clijsters.

"What she did was unacceptable. It's unacceptable behaviour under any circumstances. When you're on the court, and you are waving your racket toward a linesperson and using profanity, it's just simply unacceptable," Curley said.

"When you look at the tape, it's pretty clear that the way she approached the linesperson, with her racket and in that manner, it was a threatening manner. It certainly was."

Television commentator and former champion John McEnroe was incredulous that the rarely called foot fault penalty was handed down at such a moment, though he and his colleagues expressed dismay at the actions of Williams.

Replays could not conclusively establish whether a foot fault had actually occurred.

Serena Williams released a statement acknowledging "in the heat of battle I let my passion and emotion get the better of me and as a result handled the situation poorly."

The ending blurred a triumphant achievement for Clijsters in her first Grand Slam since coming back to tennis. On Sunday, Clijsters became the first mother to win a major since Australia's Evonne Goolagong Cawley in 1980.

With files from The Associated Press