As one achieves focus, the mind quiets. As the mind is kept in the present, it becomes calm. Focus means keeping the mind now and here. Relaxed concentration is the supreme art because no art can be achieved without it, while with it, much can be achieved. One cannot reach the limit of one’s potential in tennis or any endeavor without learning it; what is even more compelling is that tennis can be a marvelous medium through which skill in focus of mind can be developed. By learning to focus while playing tennis, one develops a skill that can heighten performance in every other aspect of life.
-- Timothy Gallwey, The Inner Game of Tennis: The Classic Guide to the Mental Side of Peak Performance (p. 83)