Mats' Musings: Frustration gets best of Serena

Editor's note: Former world No. 1 and seven-time Grand Slam champion Mats Wilander is writing exclusively for Yahoo! Sports throughout the U.S. Open. The 1988 U.S. Open champ breaks down who's hot and who's not, offering his thoughts courtesy of Lacoste.

Serena Williams' outburst on Saturday night was borne out of frustration at the way she was being outplayed, but that doesn't excuse her behavior toward a line judge.

Clearly Serena felt like this was a tournament she was destined to win, and she had every reason to think that after her dominating performances up until the semifinal.

It must have been difficult for her to understand how it was that Kim Clijsters could be playing so well after taking 30 months off to have a child.

When the critical time came and Williams was riled up by the foot fault call at 5-6, 15-30, she was just about ready to explode.

I am not defending Serena, and there can be no excuses for behaving in the way that she did. It is just not acceptable to speak to another person like that on a tennis court.

Back when I was playing we would often see that kind of thing from John McEnroe and Jimmy Connors. They would scream and curse at the umpires and you just had to stand there at the other end and take it.

There are rules in place to guard against that sort of thing and Serena will have to live with the repercussions plus any further censure that is handed down.

The most important thing she needs to do first is to offer a full and genuine apology for the way she acted.

You don't have to change your mind and agree with the officials or the decision they took, but you need to know when you have crossed the line and used the wrong words. Let us hope Serena either sees sense or gets some good advice.

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