Physical fitness is the key to the success of famous athletes. Their physical fitness begins with a desire to be better than the next athlete, as well as to be the best at what they do. This mindset enables them to prepare their mind as well as their bodies for success.
Being mentally ready helps them to do the requisite things, such as working out consistently to fine-tune their skills, maintaining a diet that supports their physical health, and adhering to team rules.
These are all similar to the care that should be put into fit relationships.