This quote from Arthur Ashe is a great tip for anyone who plays competitive tennis:
"Start where you are" means to realize exactly what the situation is, but to play the point as if this is the first point of the match without interference from what has already happened. A player cannot do anything about points in the past.
"Use what you have" means to recognize the best shots you have in your bag, the type of point that best suits your shots, and to make those your weapons and strategy for the point you are about to play.
"Do what you can" means to put forth your best effort on every point and to play within yourself, as described by the previous two parts of this quote. As simple as this sounds, this can go a long way in a difficult match.
The player who executes these ideas better is usually the winner.