New balls change the match, and hardly anybody watches for it

The fluff on a used ball adds drag and slows it down, so the ball change quietly hands the serve back its advantage.

Every nine games at most events, seven for the first change, a fresh set of balls comes out. Commentary notes it and moves on. It is one of the few genuinely predictable tactical shifts in a tennis match and it goes almost entirely unexploited by viewers trying to understand what they are seeing.

The mechanism is the nap. A new ball has a tight, compressed felt surface. As it is struck, it fluffs up, the felt fibres stand out, and the effective surface area and roughness both increase. More drag means the ball decelerates faster through the air, which means it arrives slower and drops sooner. A ball played hard for eight games is measurably slower to travel than one out of the tin, quite apart from what has happened to the pressure inside it.

So the immediate consequence of a ball change is that serving gets easier and returning gets harder. A server holding at fifteen with old balls might be facing break points with old ones a set later, and be untouchable in the game right after the change. Players know this and plan around it. There are servers who will take more risk on a second serve in the first game with new balls, and returners who will accept that the game after the change is a write off and target the games at the end of a ball cycle instead.

There is a subtler version at Wimbledon in particular. Grass is dusty and worn by the second week, the ball picks up grass and grit, and the felt on a used ball behaves differently from the felt on a used ball at a hard court event. Players talk about balls getting heavy. That is not entirely a figure of speech.

What I would like to see, and it would take a broadcaster about a week to implement, is average serve speed and average rally length plotted by ball age across a match. The pattern would be obvious immediately, and it would give viewers a real handle on why momentum appears to shift for no visible reason.

Momentum is the word we use when we have not noticed the mechanism. Here the mechanism is a tin of balls and a bit of fuzz.