The myth of the upward trajectory of the ball on the serve

While analyzing a few of my high speed videos and hundreds of videos from the pros, I begun to question the truth about the serve's upward trajectory of the ball right after contact.

From a long time ago I've heard this coaching "holy truth" about the arc of the trajectory after contact. It was said that you needed to be extremely tall (2,20 m?) to be able to hit the ball directly down.

In fact all the pros seem to hit the ball directly down on first serves. For the second serves, the great majority of players still hit the ball straight down with some exceptions where the ball stays leveled for a few frames before starting to drop.

It is very difficult to be sure about the trajectory of the ball after contact in the pro game because you need a high speed footage and the angle of the recording has to be the right one. Maybe there are a few pros who produce an upward trajectory on their second serves, but I believe that is far from being an advantage. They probably hit it with more spin but with much less speed.

Research has been made by John Yandell about the amount and type of spins that some of the pros generate for their serve, volleys and groundstrokes. For instance Sampras was found to be serving with an average spin of 2500 rpm. According to Yandell, it seems to be impossible to generate perfect topspin since there is always a side spin component on every serve.