Here's another good video of Nadal and Robredo playing some practice points. The video is shot at about court level and pretty close to the players. The players appear to be hitting pretty seriously.
Limpin is correct that the perspective can be deceiving, but one way to deal with that is to draw an imaginary line between the two players heads. That's about six feet. If the ball at any point in it's flight goes above that line then it has to be more than six feet off the ground, regardless of angle, depth, etc. If is stays below then it never attains that height.
There are definitely some defensive shots that clear the net by 6 or more feet. There are also some topspin backhands that Robredo hits from pretty far behind the baseline that appear to over 6 feet. But a lot of the shots, especially when they really hit out on the ball, don't go above that 6 foot line between the heads of the players (i.e. 3 feet over the net).
I saw the same thing on the other videos that I posted.