Cyclists, Runners, they train every day. 2 days rest is heaps to them. I mean Marathon runners run the equivilent of 4 marathons in training a week, the top ones. They run 160KM a week.
Tennis players don't just run. If that's the only thing you see that makes tennis players tired, you haven't played tennis. Djokovic ran "only" 11km in the AO Final, and he ran about 10km in the semi two days earlier. I can run 11km in less than couple of hours and recover by the afternoon. The difference is with tennis players is – that's 11km of stopping, starting, changing direction, sudden bursts all the while aiming, anticipating, reacting, planning, jumping and swinging a racket with almost as much power as a boxer. It's difficult enough to run 11km at a leisurely marathon pace but add all those other things to it and you'll be exhausted by the halfway point.
The other main issue is the type of sport you're comparing tennis to. Marathon running is more akin to golf. Tennis is more like boxing, in that, it's gladiatorial, you're directly affected by what your opponent does. An opponent can force you to run, or run backwards, make you play out of your comfort zone or simply overpower you for a win. In marathon running, if an opponent runs faster, you can run faster as well or maintain your speed and catch up later, you're still battling, and in essence dependent on, the terrain – same as golf where if an opponent shoots a better round, you're chasing after his total but it's still you vs the course.