Well I was recently at an ATP Challenger tournament in my country, where Lu Yen-hsun was playing in the finals. I was actually pretty surprised that the balls they hit were pretty close to the net cord, perhaps only about 20cm net clearance? In fact, the balls I usually hit on a rally are moonballs in comparison
One thing I noticed is that the ball is usually past its highest point by the time they reach the net.
Well LYH is a top 40 player (I think?) so I don't know if it applies to the guys higher up as well? But since these guys play the likes of Federer, Nadal and Djokovic I would say they would play rather similarly too?