Do you play on red clay? unless I'm trying to hit like soderling/berdych and aiming for the lines and corners most of the fitter guys that I play can run down shots effectively.. You need topspin and slice to mix things up both in terms of timing as well as height of your ball.
It's not just about speed and flatness of your strokes that allows you to take time from your opponent, movement and placement matter a lot.
Disclaimer: I'm NOT a coach...
Real tennis: https://www.youtube.com/watch?v=yDqnkLJ9BtM