The loudest cheers during a tennis match typically come after long rallies, the seemingly endless exchanges of forehands, backhands and grunts.

Fans live for long points. But exactly how much action is there in a tennis match? We took a stopwatch and timed two matches at the U.S. Open last week to find out. The answer: Not as much as you'd probably think. In the two matches we studied, only 17.5% of the time was spent actually playing tennis.

There's plenty of down time in tennis. Players take breaks at changeovers and between sets. They argue and challenge rulings. But most of the down time was spent between points, when players strolled around, toweled off and bounced balls before serving.