Never Give Up, No Matter What
Once playing a close friend in game, both equally matched, I was being dominated trying to think about every possible move, getting frustrated whenever it failed.
Soon enough, it was 5-0. After a while I stopped caring about how I hit, what I was doing wrong. etc.
The more I let my body play on its own, with little thought, the better the result. The score at the end turned out to be 5-7, where I won.
Just goes to show you that tennis is mostly a mental game and don't give up on it.