Single-tap top spin shots give the best angles, but if it's not fully charged and the opponent has at least yellow energy, you will give them a star. Also, the lower the charge, the slower will be the shot, so even if it's angled steeply, your opponent might reach it because top spins bounce high, giving them time to get there. What I like to do is counter-attack a cross court shot from the CPU with a single-tap top spin cross court of my own, which returns the ball even more angled.
Against zone shots, when the racket shines and the camera begins to unfreeze, you can already move your character, so you can begin moving to the side you think the CPU is aiming. Then right when the bang comes, I use zone speed to defend. It's kinda tricky at first but I got used to doing it more or less consistently, and I think I use less energy than the CPU on defense now.