By Tom Perrotta

Let's start with something every tennis player can agree on: Foot faults are pathetic calls.

The point of the foot fault rule, of course, is to prevent the server from gaining an unfair advantage by striking the ball closer to the net. For recreational players, it can be an important rule, but it's difficult to enforce since there are no line judges. In the pro game, the foot fault is a harmless sin. When pros foot fault, they do it by fractions of inches, distances so small that no unfair advantage is gained. And they do it by accident. No pro polishes a service motion over two dozen years and then decides, in a moment of duress, "I'm going to sneak in for this ace and hope no one catches me!" Marat Safin put it best at last year's U.S. Open, when a foot fault sent him into a tizzy.

[It] doesn't help me to serve better," Safin said. "It's stupid rules that somebody made in, I don't know, 1850, and now they give me the problems with these things."

Safin's foot fault came in the fourth set of a first-round loss, and by now few people remember it. But when the last word on tennis is written, the Serena Fault is going to be as famous as the Serena Slam, if not more so. It came at 15-30 in the second set of last night's semifinal against Kim Clijsters. Williams was in trouble. Her serve, usually her best weapon, had traveled long and wide all evening long, and Clijsters—who moves better than any mom on earth—was closing in on the biggest upset of her comeback. Williams missed her first serve badly. And then the call: "Foot fault!" Which meant double fault. Which meant match point.

Williams showed little emotion at first. She turned to her right, called for a ball—and then she lost it.

"I swear to God, I'll [expletive] take this ball and shove it down your [expletive] throat, do you hear me?" she said as she walked toward the lineswoman who had made the call. She shook her racquet and continued her tirade, but the rest of her words were more difficult to hear, save a few profanities. Williams had already received a warning, for cracking her racquet at the end of the first set. Once Brian Earley, the tournament referee, stepped onto the court, it was clear Williams wasn't going to escape without a point penalty. On match point. Game over.

Williams had every right to be angry, because this was an awful, foolish, atrocious, silly call. Clijsters had played so well, so intelligently, and she didn't need any help earning a match point. The small crowd assembled inside Arthur Ashe Stadium endured a day of rain, as did fans watching on television at home. Lots of money—and a lot more pride—was on the line, and as replays showed, this was hardly an egregious foot fault, if it was even a foot fault at all. There's a difference between being a conscientious official and being officious, and this lineswoman certainly crossed that line. She inserted herself into a match that should have been decided—frankly, that was moments away from being decided—by two first-rate athletes. People pay to see athletes, not a lineswoman.

When Williams and the officials convened at the net, there was a suggestion that Williams had said she would kill the lineswoman. Williams looked stunned by the accusation and those words were not audible in replays I watched. Was she angry? You bet. Was she threatening? That's a loaded word, and one that needs to be used carefully. Does anyone really believe that Williams would actually shove a ball down a lineswoman's throat? I don't. I can understand how the woman was a bit nervous at the time, and certainly embarrassed. But threatened? Sorry, I can't see it. This was still a tennis match, not a heavyweight fight. Not long after the incident, Williams was giggling with her sister outside the women's locker room, and she was composed and funny in her press conference.

"I've never been in a fight in my whole life, so I don't know why she would have felt threatened," Williams said. She added: "I didn't think I would get a point penalty. I didn't think about it."

Of course, other players have said worse, or at least equally nasty, words and not received even a warning. Just yesterday afternoon we were treated, for what must be the millionth time, to Jimmy Connors versus Aaron Krickstein in 1991. These were among Connors' choice words for umpire David Littlefield on that day. "Kiss me before you do that to me." "You're a bum." "You're an abortion." Connors got away with it. By that standard, one might reasonably conclude, Williams was robbed.

But there's a difference in this case. Connors attacked a chair umpire, who can fend for himself, issue warnings, take away points as he sees fit, and even default a player. Williams abused the lineswoman, who can't do anything but tell the chair umpire what was said to her. As a chair umpire, you have to protect your employees from verbal assaults. Chair umpires are responsible for the well being of those who work for them, and so the threshold for a warning is lower. Without a doubt, this penalty was warranted.

Luckily, none of this changed the outcome of the match. Clijsters was going to win; she had outplayed Williams all night. Did it sour the ending? Sure, but that won't matter one bit if Clijsters wins the title tonight, as she should.

And besides, something good is bound to come of this. Remember the last time Williams was in the middle of a U.S. Open controversy? That one involved an ill-advised overrule by a chair umpire against Jennifer Capriati in 2004. So bad was the call that Hawk-Eye, the instant replay system, became a permanent part of the Open, and then the rest of the sport. Maybe this time, tennis officials will update the foot fault rule. Or at the very least, begin to use Hawk-Eye for that, too.

Tom Perrotta is a senior editor at TENNIS magazine.