Long rallies tire the eyes before they tire the legs

Visual tracking degrades under fatigue faster than the muscles do, which is why late errors look like mistiming rather than exhaustion.

The classic image of a player breaking down late is legs. Heavy steps, slow to the corner, a forehand hit off the back foot. The errors that actually decide long matches look different from that. They look like a ball struck slightly off centre, a fraction late, on a shot the player was in perfect position for. That is not a leg failure. That is tracking.

Following a ball travelling at speed requires a smooth pursuit eye movement plus a series of predictive saccades, and both degrade under general fatigue and under the specific fatigue of having done it several thousand times in an afternoon. The consequence is not that the player stops seeing the ball. It is that the estimate of where it will be at contact gets a little noisier, and a little noise at the contact point is the difference between a clean strike and a frame.

You can watch this happen. Late in a long match players start taking the ball later, standing further back, and hitting with more margin, all of which are unconscious compensations for a less reliable read. Commentators call it playing within yourself. It is closer to a player quietly buying time to see.

Which suggests conditioning for late match performance should include something other than running, and here I am on thinner ground because the evidence for visual training in sport is patchy and full of expensive nonsense. What seems defensible is that practising at genuine match intensity for genuine match durations trains the whole system, eyes included, and that hitting for two hours at a comfortable pace does not. Most practice is the second thing.

There is also a simple environmental piece. Light quality, contrast against the background, whether the player is squinting into low afternoon sun on one end and not the other. All of that changes the load and none of it appears in any fitness plan I have seen.

The player who is missing at three hours has usually not stopped moving.

They have stopped seeing the ball quite as early as they were, which is the same thing arriving from a direction nobody looks.