Then there are guys who can go all day in the practice room but tire in the first period of a match. It is a weird thing. I don't remember the lady's name, but there was a marathoner from England, I believe, who would win almost every marathon she entered. But whenever it came to the Olympics, she would tire out for some reason. I don't even think she could finish the last Olympics she was in. It is really a strange thing to see people like that.
It really is amazing how debilitating the mind can be to an athlete. Mental pressure to perform/succeed can certainly lead to a chemical imbalance in the body leading to its opposite result.