Serena Williams’ meltdown in the U.S. Open semifinals reminds us exactly how much emotion goes into high-level competition.

We don’t always see that emotion, because pro athletes get where they are precisely because they manage their emotions so well.

When batters go to the plate in crucial situations, they must fight the pounding in their heart and the urge to grip the bat too tightly. Field goal kickers, free-throw shooters, you name it: They all must let their bodies function smoothly amid high stakes. And most of the time we don’t see a dramatic sign of this struggle.

But Williams really lost it when an official called a foot fault on a crucial point near the end. Williams’ tirade then cost her match point, not to mention some fine money and a ton of respect.

On the other hand, Kim Clijsters provided her own study in the mental side of competition. In becoming the first mom in nearly three decades to win a tennis major, Clijsters showed perspective. Tennis is still important to her, but not nearly a important as her daughter. The secret of doing well in high-stakes competition is to make things important, but not TOO important. Now that she has a daughter, there seems to be little danger that Clijsters will look at tennis as the biggest thing in her life.