Players do not slow down first, they stop stopping

Repeated hard deceleration is the first quality an aging body loses, which is why the second week of a long event ends careers.

Ask a thirty four year old what he has lost and he will tell you a step. Watch the tracking and his top speed is often close to what it was. What has gone is the brake.

Getting to a wide ball is a sprint and getting back is a stop, and the stop is the expensive half. Deceleration is produced by muscles contracting while being stretched, generating high force in a short distance, and the tissue involved is loaded near its limit each time. That kind of contraction also causes the most microscopic damage, which means it is the movement with the longest repair time and the one that accumulates a backlog across consecutive days.

Younger tissue clears that backlog overnight, more or less. Older tissue does not, and this is the honest mechanism behind the pattern everybody notices without explaining. A veteran can play a superb match on Monday. He can frequently play another on Wednesday. What he struggles to do is the fourth hard match in nine days, because the deficit from each set of hard stops has not been cleared before the next set is added.

Which is why the second week of a two week event is where the reputations get made and unmade. The first week filters on quality. The second week filters on recovery of a very specific capacity, and it is the capacity that ages earliest.

The knock-on effects are visible if you look for them. The aging player takes the ball earlier, because less running is required. He shortens rallies deliberately. He plays more first strike tennis and gets called tactically smart, which he is, but the tactic is chosen by his tendons. He also picks his tournaments differently, avoiding back to back events on different surfaces, since a surface change forces a new set of stopping patterns onto a system that has no spare capacity for learning.

None of this is about willpower and very little of it is about conditioning in the ordinary sense. You can train the brake, and everybody does, and it still fades. The interesting question is not why a player at thirty five cannot go five sets. It is why he can, once, and then not again on Thursday.