A tired forearm squeezes harder, and that is the problem

Grip force rises as fatigue sets in, which raises the shock passed into the elbow at exactly the wrong point in a match.

Nothing on a tennis court gets tighter as it gets more tired except the hand.

Grip force is not fixed. A fresh player holds the handle loosely between shots and firms up only in the moments around contact, which is the efficient pattern and also the protective one. Fatigue erodes that timing. As the small muscles of the forearm lose their ability to produce force quickly, the body compensates the only way it can, by holding on harder for longer. Watch the hand of a player in the third hour and you will see white knuckles between points.

That change matters mechanically. A racket held loosely can rotate a little in the hand on impact, and that rotation is a way of shedding energy. Clamp the handle and the frame cannot move, so the shock that would have been absorbed by a small slip is passed straight into the forearm and up to the elbow. The tighter the grip, the higher the peak load on the muscles that attach to the bony point on the outside of the joint.

There is a second contribution from string tension here, and the two stack in an unhelpful direction. A stiff string bed already produces a shorter, sharper impact. A tight grip removes the last cushion in the chain. Put a fatigued player, a polyester string bed at high tension and an off-centre ball together and you have assembled every ingredient of an elbow injury in a single stroke, at a moment in the match when the player is least likely to notice the warning ache.

The reason I keep returning to this is that it explains an odd pattern. Elbow trouble in club players rarely starts in the first hour of a session. It starts at the end, in the last set, when technique has quietly deteriorated and the hand has taken over. So the advice to reduce playing time is directionally right for a boring mechanical reason: it removes the segment of the session where the shock per shot is highest.

The counterintuitive fix is to change equipment for the end of the match, not the start. A lower tension, a slightly heavier and more stable frame, a thicker grip that requires less closing force. All of them shave the peak at the moment the arm can least afford it, and none of them will make anyone hit the ball better.