Have you ever noticed that often, when a professional player is about serve, he will collect three balls, examine them and then throw one away. So what is happening here? It all comes down to hair. New tennis balls tends to have hair which is smoothed flat, whilst a ball that has had more play will be fluffy.
Tennis players may check three or more balls before serving so that they can select one smooth ball and one fluffy ball. They like to use a smooth ball for the first serve because the hairs are flattened down and therefore the ball should travel faster than an older ball. If the first serve goes astray, the player will use the fluffier ball for their second serve. Although these move more slowly, they are easier to control and so the player is less likely to concede a double fault.
Here you can see Rafa Nadal selecting the balls ready for his serve
If you have a Marbella tennis lesson you will be able to learn other techniques for improving your service game.