There's a really famous book about this that's good reading and applicable to all sorts of things in life called "The Inner Game of Tennis". Nice 2x-able 10-minute synopsis here:
"Conscious trying, directed by the conscious mind, often produces negative results. You know this instinctively by the fact that when you're playing at your best, you are not thinking in technical way about your shots. You are a fluid unity of mind, body, court and racket. You are in a state of flow."
Quiet the mind.
Through not being attached to the fruits of victory, you paradoxically become free to play the game for itself in a more relaxed and powerful state of mind. Through non-attachment, getting a 260 is more likely.