The ball changes more between games than the players do

New balls every nine games reset the aerodynamics mid-set, and almost nobody watching accounts for it.

A tennis ball is a felt-covered pressure vessel that starts losing its properties the moment it comes out of the can. Within a few games the nap has fluffed up, which raises drag and slows flight. A few games after that the felt is worn flat in patches, the internal pressure has dropped, and the same swing produces a different shot.

Then somebody hands both players three new ones and the match restarts physically without restarting on the scoreboard.

Watch a server who has been struggling to hold and count the games. The new-ball game is the easiest hold on the sheet, routinely, because a fresh ball is faster through the air and sits lower off the bounce, which shortens the returner's reaction window. Watch the same server two games later and the aces have gone. That is not confidence. That is fluff.

The returner has the mirror problem. Heavy topspin needs the felt to bite, so a grinder gets a better ball at exactly the moment the server gets a worse one, which is why the middle of a ball cycle so often produces the long rallies and the break points. Some players talk about this openly. Most commentary treats it as momentum.

Then there is the bit that genuinely annoys me. Ball specification varies by tournament, by supplier, by the altitude the event is played at, and even by how long the cans have sat in a hot storeroom. Players move from one event to the next and find the ball they are hitting is meaningfully slower, or heavier, or fluffs faster, and the tour has spent years being asked to standardise it and has not.

The case against standardising is that variety is part of the sport, and I have some sympathy. Different surfaces, different conditions, adapt or lose. Fine. But surface variation is visible and understood, and the ball variation is invisible and blamed on the player. A guy who has just lost in straight sets does not get to say the ball was thirty grams of fluff heavier than last week without sounding like he is making excuses.

If you want one number that would improve tennis analysis more than any other, it is not a spin rate or a serve speed. It is ball age. Log the game number in the cycle against every point and a lot of what we currently call form starts to look like equipment.