A tennis match at a competitive level is a high volume of quick movements, mostly side to side. It wears you out a lot. One of my old tennis coaches uses to do a conditioning workout where he'd take a shopping cart full of balls and hit 90 of them into opposite corners of the court and if I failed to get one or missed hitting it in, the counter would restart (or something like that). I remember I pretended to enjoy this drill because I thought that would make him stop doing it lol.
I think Dustin Brown could run a fast mile.