The speed of the tennis ball during the serve can make or break many games. A fast serve is an advantage, most players average between 70 - 120mph. The professionals are much faster with the record at 156mph held by Ivo Karlovic.
Velocity
In tennis, velocity includes both speed and direction of a serve. Just as it is important to be fast, it's equally important to aim the serve to the right spot on the court, or you will miss and get a foul,