Couldn't have said it better myself Jack Romeo.
He doesn't have the flexibility of Djokovic, the footwork of Federer or the willingness to throw his body all over the court like Nadal, but I believe he is the most 'explosive' athlete among these four players, especially in terms of vertical leap and raw acceleration. Of course none of us have definitive proof in front of us (tennis isn't athletics!) but this is my inclination.
Funnily enough, I was watching this exact video with a friend on Friday night while having a smoke
The point at 3:20 is mind-boggling, watching Sampras accelerate to hit the backhand is like a blur... had to be replayed several times over I recall..
Imagine Nadal in the same position - we have seen him sprint the full width of the court and curl a forehand down the line many a time, but it is usually made on the run, skidding into the shot, with his arm doing most of the work. What gets me about the Sampras clip is that he seems to get there in enough time to actually stop himself and plant both of his feet before hitting the backhand.