When playing a match, you always want to keep your opponent guessing. You want them to be off-balance and uncomfortable. You want them to be thinking to themselves how they just can’t get a rhythm, and to be frustrated that they are hitting errors. You don’t want to play to their strengths. Most people will set up the ball machine to hit the balls landing on the service line with moderate top-spin, bouncing into their strike zone, and then they practice hitting low flat impressive looking shots over and over again. But they don’t practice footwork, and they don’t practice variety – either in hitting or returning.
So if you are playing a one dimensional player, or someone who has poor footwork, it can be especially effective to hit them balls that are irregular and not something they practice. That’s not to say that you should totally change your game and stop hitting as you normally do, but when you find yourself in a neutral rally where you cannot attack easily, simply try changing up the pace of the rally. Hit a high looper or a low slice. Add or take away power from your shot, and hit something neutral but different back.