An average tennis match in the men's singles category at the U.S. Open in 2014 was two hours and 44 minutes. Less than 20 percent of that time was spent with the ball in play.
The reason that a tennis match takes longer is because there are a lot of breaks in the game, including when the players change sides or at the end of each set. Also, there are breaks to challenge rulings. Most of the time, however, is merely time spent between scores. The Wall Street Journal also timed a women's doubles match, which lasted one hour, 26 minutes. According to the report, the ball was in play less than one-fifth of the time, but it was in play a higher percentage of the time compared to the men's match.