Tim Gallway's book 'The Inner game of Tennis' first published in 1975 still stands the test of time in and amongst all the modern thinking around sport psychology.
I often refer to his definition of 'performance':
Performance = potential - interference
'A tennis player first confronts in Inner Game when he discovers that there is an opponent inside his head more formidable that the one across the net.....The Inner Game is that which takes place in our mind, and is played against such elusive opponents as nervousness, self-doubt and lapses of concentration. It is a game played by your mind against its own bad habits. Replacing one pattern of mental behaviour with a new, more positive one is the purpose of the Inner Game.'
'It takes years to change behaviour if that's what you're looking for. But behaviour comes out of how a player sees things. If he sees a tennis ball as a threat, he swings as if he's defending himself, and he does 33 wrong things. In this way, you can make radical changes in performance with only a few sentences on perception. See what he sees before you start coaching.'