It depends on the player and the surface, but the claims of 4 or 5 feet over the net as an average, which means approximately 7 to 9 feet in the air when it crosses over the net, are not accurate for most hard court play.
I mostly watch tennis in person at Indian Wells, and usually the average net clearance is a lot less. Some players tick the net a lot which gives you an idea of the net clearance. I've seen James Blake warming up and he hit like a fifty shot rally and most of the balls were probably within a foot of the net.
I can well believe the net clearance is higher on clay. And some players like Nadal will hit heavy topspin defensive shots that do go high over the net.