I recently changed from hitting inconsistent flat serves to consistent topspin serves. I modelled Sampras' serve. My service action is now pretty similar to Sampras', with similar knee bend, back arch and arm action, but I am unable to generate enough power and accuracy to ace people. I find it harder to place topspin serves than flat serves, even though it's supposed to be the other way around (I think it's because instead of swinging the racquet at my desired location, I have to swing my racquet left to right all the time). And unless it's on the very edge of the service box, it doesn't give me service winners (lack of power). How did Sampras give his topspin serve so much power? And how do you place topspin serves?