A mind that is not under control is a mind that makes mistakes. Mistakes that could lose a game. Extreme stress causes a wild mind. Stress is a helpful tool when used correctly; when not used correctly, stress can cause you to lose a lot of games.
I think you don't have to be an athlete to get "in the zone." Students know it when something they have been struggling to learn all at once comes clear and the answers flood in. I had that experience when I was a little kid, flummoxed by mathematics.