A new ball and an old ball are not the same sport

Seven games of wear change how far a shot travels, and the rulebook resets that in the middle of a set.

A tennis ball is a pressurised sphere covered in fabric that starts life smooth and compact and ends it fluffy and slow. The rules replace them on a fixed schedule, which means the physical properties of the object being hit change substantially across a set and then reset without regard to who is serving well.

What actually changes is drag. As the felt lifts, the effective surface area and roughness increase, and the ball decelerates more through the air. It also gets slightly heavier as it picks up moisture and court dust. So a shot hit with identical racket speed lands shorter and sits up more. Spin bites more too, because there is more surface for the strings to grab, which is why heavy topspin players tend to look better with older balls and flat hitters look better with new ones.

That is not a small effect and it is almost never discussed as a tactical variable. Players feel it. Commentators mention new balls and move on.

Consider what happens at the changeover where fresh balls arrive. The server who was struggling to hold now has a ball that flies faster and skids more, which favours the flat first serve and punishes the kicker whose action relied on grab. The returner who had settled into a rhythm of stepping in on shorter balls now finds everything arriving quicker and deeper. Both players receive a small handicap swing that nobody chose and that lands wherever it lands.

I think the sport should be more honest that this is partly a lottery. A player two points from losing a set who receives new balls at the right moment has been handed something. We accept it because it has always been that way.

There is an equipment argument sitting underneath. Balls behave differently at different altitudes and in different humidity, and tournaments select from different manufacturers with meaningfully different characteristics. A player arriving from a tournament with a lively ball onto a court with a heavy one is not adjusting a detail. They are recalibrating every rally length they know.

Watch the fifth and sixth games after new balls, when the fluff has come up but the pressure has not gone.

That short window is when the best rallies happen, and it is entirely an accident of manufacturing.