Stajus,
right now, in TE, it's about as tiring to run 5m at full speed than doing a acceleration. Roughly put, that means that hitting 20 accelerations is as tiring as running 100m. (with 100% speed & stamina)
Now, I'm sure you can hit 2000 times without getting tired (20'000, I begin to have some doubt though

).
In TE, you don't really get tired by hitting the ball either. Only the accelerations are noticeably tiring.
I don't know your level in real life, but there's a whole word between hitting a ball at 90km/h like a decent club player does and doing accelerations at 135km/h . That's 50% more speed. As you learned in Physics at school, that's 2.25 times more energy put in the ball. But to obtain this additional energy, you have to push & more or less jump on the legs ; most of the energy is lost in the jump & comes back on the legs during the descending phase ; thus, the energy efficiency is way lower when hitting at 135km/h. So to achieve this, you have to use likely something around 4 or 5 times more energy than hitting at 90kh/m (that works same with 150km/h & 100km/h

).
And that's not all. Doing accelerations requires perfect feet positioning, from the very beginning of the waiting to the actual preparation of the shot, and this positioning needs to be fast. Thus this also requires a lot more energy than simply hitting the ball...
Personally, my stamina is quite bad, but I can jogtrot & hit balls for hours on a tennis court ; I can even still sprint once per game after hours of play. But what drains my energy is doing acceleration...
And if you look the pros, most of times, they don't run that much, just a few meters, and usually, far from their full speed. Running is not much an issue in tennis. As long as they don't get cramps, players still run at a decent speed in the 5th set...
about serves, there's already a topic for that matter. But don't worry, I'm not planning to change anything soon.