It was Serena Williams' mouth, not her footwork, that cost her an important match Saturday.

During a semifinal match at the U.S. Open a line judge called a foot fault on Serena during a match point serve. The tennis pro missed the serve before, which meant a double fault and ultimately losing the match to challenger Kim Clijisters.
 
The call may be have been ill-advised because video of the serve in question didn’t clearly show whether Serena’s foot touched the baseline. However, the argument quickly escalated past that issue when Serena started arguing with the line judge.
 
Serena began swinging her racket and shouting at the judge while liberally using a few colorful words. "I swear to God I'm [expletive] going to take this [expletive] ball and shove it down your [expletive] throat, you hear that? I swear to God,” she said, according to ESPN.
 
The judge, who reportedly felt threatened, called over an umpire. After a quick conversation with Serena at the net, the umpire charged her with unsportsmanlike conduct. The game was then called in Kim’s favor 6-4, 7-5.
 

 
Serena still insists that she didn’t threaten the line judge, and claims that she can’t really recall what she said during the outburst because she was “in the moment.”
 
“I’ve never been in a fight my whole life, so I don’t know why she would have felt threatened,” Serena said. “If she called a foot fault, she must have seen a foot fault. I’m not going to fault her for doing her job.”

-- Sonya Eskridge

Here’s more:
Serena Williams puts it ‘On the Line’
Venus and Serena go pro football
Serena stomps competition at Wimbledon