Serena Williams was fined $10,500 by officials at the US Open and could face further sanctions after an investigation was launched into her angry outburst during her semi-final defeat by Kim Clijsters.

It is the biggest fine handed out to a woman player since records started being kept in 1990, according to the ITF.

"The maximum permissible on site fine of $10,000 has been levied for unsportsmanlike conduct against Serena Williams for her on court behaviour during the women's semi-final," the tournament referee said .

"Ms Williams will also be fined $500 for racket abuse which took place at the end of the first set of the same match."

Trailing 4-6 5-6 15-30, Williams launched into a second serve but the lineswoman sitting at the baseline held up her finger to call her on a foot-fault - meaning the American had served a double-fault to go match point down.

Astounded by the verdict, Williams walked over to the official screaming. She waved her racket ominously in the lineswoman's direction and then shook a ball in her clenched fist as she threatened "to shove it down" her throat.

Having already received a warning for smashing a racket, Williams was handed an automatic point penalty for a second violation which ended the match, giving Clijsters victory.