Well, I think you made a wrong comparaison with cycling and running.
I will just answer on one thing : in tennis, you can be tired physically of course, but also mentally. You need to be sooo focus at the ATP level, on each point, you have to come with a plan, you have to be ready to adapt it, handle the pressure in the decisive moment, accept the frustration... tennis is such an exhausted sport in the mental department. A 5 sets' match takes you a lot of energy in the mental department because tennis in more emotional than others sports.
You think running 42.2km or cycling for hours and hours and hours and hours and hours isnt mentally tiring?