The following comes from "Coaching The Mental Game" by H.A. Dorfman:
For an athlete, relentless behavior is an expression of his will to win. An expression of mental toughness. The primary reward is, once again, in the behavior, not in the result. But invariably, a "way to winning" is in being persistent. To persist in doing the right thing. In executing tasks well and responding to adversity with courage and determination.
John Cheever called it the discipline of continuousness. To the relentless pursuit of what is challenging or difficult.
Coaches should concede that this kind of toughness can be learned by so-called soft athletes if the subject is addressed and reiterated regularly -- as instruction, rather than ridicule of an athlete's softness.
"Victory belongs to the most persevering."
-Napoleon Bonaparte