The clock between points does not care how long the rally was

Recovery of the muscle fuel that powers a sprint is time-based, so identical rest after unequal work is not equal at all.

A twenty two stroke rally and a two stroke ace are followed by the same twenty five seconds. That is the structural oddity at the centre of modern tennis, and it decides more matches than anyone credits.

The fuel that powers the first few seconds of any all-out effort is a small compound stored inside the muscle, and it is used up quickly and replaced only during rest. Refilling it takes time, and the curve is steep at first and slow afterwards, so a short pause returns a good share of it and a long pause returns nearly all. Note too that the refilling rate does not speed up because the preceding effort was harder. It just has more to replace.

Follow that through a set. A player who wins his points in three strokes drains a fraction of the store each point and gets it back easily. His opponent, playing twelve stroke rallies, drains far more and gets the identical window. The deficit compounds. By the third set the second player is starting each point from a lower level and cannot produce the same racket speed or the same first step, and he does not feel this as breathlessness because the aerobic system is coping fine. He feels it as heaviness.

Anyone who has watched a long baseline match knows the shape of the collapse. Not a gradual decline. A cliff at a specific game, usually right after a very long game, because the deficit crosses a threshold rather than accumulating smoothly.

Then the injuries, which follow the same clock. The first quality to degrade when this store runs low is the ability to produce force quickly, and the movement most dependent on that is stopping, not going. A player still gets to the ball late in a match. He just cannot decelerate the way he could, and the muscles at the back of the thigh and around the hip are the ones asked to make up the difference.

There is a reasonable argument that the shot clock made this worse by removing the informal extra seconds that used to be available to a tired player. It did. Whether that is a fair sporting change is somebody else's article. What is not arguable is that the punishment lands hardest on the player who just did the most work, which is a strange thing for a rule to do.