Keep your eye on the ball. That’s the mantra for tennis players, from beginners to whoever lands in the finals at this year’s United States Open.
But each ball will be seen only briefly because in tournament play, six balls are used to start a match, then ditched after seven games; for the rest of that match, the balls will be replaced after every nine games. (The Open generally stocks about 100,000 new balls and goes through about 70,000 each year.)
Those life spans, punctuated by the chair umpire’s call for “new balls, please,” are necessarily brief because the balls take a beating.