Pro tennis players often pick three or four balls before they serve but only use two. This is actually a tactic to increase their chances of winning a point. For the first serve, players want a ball which is smooth, without any visible bits of felt coming loose. This is because the ball will fly through the air faster so it makes it harder for opponent to return. For the second serve, players want a ball which is a bit more fluffy. This is slightly easier to control, reducing the chance to hitting a double fault, as it grips the tennis strings better.