The $10,000 penalty — not quite 3 percent of the $350,000 in prize money Williams earned by reaching the semifinals at Flushing Meadows — is the maximum on-site fine that can be issued for unsportsmanlike conduct at a Grand Slam tournament.
The U.S. Open said in a statement that the Grand Slam Committee Administrator will "determine if the behavior of Ms. Williams warrants consideration as a major offense for which additional penalties can be imposed."
Williams, who was the defending champion, also was docked $500 for smashing her racket after the first set of what became a 6-4, 7-5 loss to Kim Clijsters on Saturday night.
Two points from losing in the second set, Williams faulted at 5-6, 15-30. On her second serve there, the line judge called a foot fault, making it 15-40 and giving Clijsters a match point. Williams began berating the line judge and a penalty point was awarded to Clijsters; because it happened to come on match point, it ended the semifinal.
"Last night, everyone could truly see the passion I have for my job. Now that I have had time to gain my composure, I can see that while I don't agree with the unfair line call, in the heat of battle I let my passion and emotion get the better of me and as a result handled the situation poorly," Williams said in a statement released Sunday by a public relations firm.
"I would like to thank my fans and supporters for understanding that I am human and I look forward to continuing the journey, both professionally and personally, with you all as I move forward and grow from this experience."
The chairman and CEO of the women's tennis tour, Stacey Allaster, issued a statement calling Williams' conduct Saturday "inappropriate and unprofessional."
"No matter what the circumstances, no player should be allowed to engage in such behavior without suffering consequences. I have spoken with the USTA about this matter and I agree with the action they have taken," Allaster said.
Williams and her older sister Venus are scheduled to play in the women's doubles final Monday. Venus got in some work on a U.S. Open practice court Sunday; Serena wasn't with her.