On a professional court, the clock does not decide when tennis balls are replaced. A fixed count of games does. Tournament officials track the scoreboard, not a stopwatch, because the key variable is how much impact and friction each ball has absorbed in live play.
Each rally strips and compacts the felt, altering surface roughness and, with it, the drag coefficient that governs airflow around the ball. As fibers shear under repeated collisions and high spin, the boundary layer over the ball changes, shifting lift, trajectory, and bounce height. Two matches that last the same number of minutes can produce very different cumulative impacts on the balls if one is dominated by short points and the other by heavy baseline exchanges.
A fixed game interval gives organizers a predictable maintenance protocol that maps directly onto rally volume, serve frequency, and average ball speed, all of which drive material fatigue and elastic deformation. Time on the arena clock, by contrast, is a poor proxy, bloated by changeovers, towel routines, and video reviews that do nothing to the ball’s polymer core or felt cover.
