It is true that tennis requires skill, but without stamina, even if you are the most talented tennis player ever, you'd be nothing.
Example: Where would Nadal be if he did not have the fitness and stamina to chase every single ball?
Not only the match stamina and endurance.
Two equal guys technically - one of them can train 12 hours per day on high intensity B2B2B2B, the other one can do 6 hours and needs more rest.
Obviously one of them will have immense advantage in the match later on.