One must put oneself in the position where one need to concentrate for a sustained time. One must persist. One must guide one's mind to go where one want it to go. With practice one's mind becomes used to concentrating, and the thinking becomes cleaver. If ones brain sits about idly, without ever working up sweat, ones mental muscles becomes sluggish.
People get mentally out of shape when they stop challenging their minds.
It also happens when one confines thinking to a small range of interest. When people stop having fun using their minds to search out new ideas by experimenting, by playing around with new possibilities, they become mentally rigid. Hence have the time, patience and determination to develop a conditioned mind. It is this combination of a conditioned mind and physical fitness that help excel in the game of both tennis and life.