Tennis allows a player two chances to land a legal serve in one point. The first serve is usually faster and thus has a high win percentage. The second serve is relatively slower and can be better read by the returner.
A wrong first serve results in a fault. A wrong second serve leads to a double fault which means that the server loses the point. In order to minimize the delay between two serves, tennis players carry an additional ball in their pocket. Earlier, tennis clothing was not suitable enough that could carry the ball in a pocket. In those days, players used to keep the extra ball in their hand. But now that we have a provision of pockets in our clothing, it is easier for the players to carry the extra ball.
Right before a player serves, he or she would ask the ball boy or girl for the balls. The server will keep two or three balls and return the remaining balls to the ball boys or girls. If the first serve lands wrong, the player will draw the extra ball from his pocket and make the second serve. This also reduces the chances of a lapse of concentration and the player can serve again in his or her rhythm and flow.
In case the first serve lands right, the extra ball stays in the players’ pocket. In fact, modern clothing has been so intricate that the extra ball does not hinder the players’ movements on the court.
Thus, tennis players put a ball in their pocket to avoid any unnecessary delay and maintain their service rhythm.