Next amazing picture. 4th, 5th, 6th frame - why the elbow is so high? Why it doesn't move? Because Sampras uses INTERNAL ROTATION OF UPPER ARM as a fundamental source of energy and on these frames we see the artifacts of this method.
Generally, if you think about tennis strokes, do not think about angles, velocities etc. Think in terms of energy and its transfer - it is much easier to understand tennis physics this way.
So, here you have the real physics of tennis serve. Should I publish my account number now?