A great number of tennis players have roughly the same level of talent and resources available to them. Yet, some of these players become successful stars and other fade into the night. Why is that? The number one reason is motivation.
Motivation (also read: Be like water and not like ice) for success pushes players out of their comfort zones and to greater heights. They are forced to go beyond themselves to gain whatever edge they can, and being highly motivated is the only way of accomplishing such difficult tasks.
Of course, this is all a mental challenge, not a physical one.