Players learn to react to an opponent’s motion as soon as they are committed. But some players commit early, other late. You can often see where an intermediate player is hitting his shot before he even starts his forward swing, while you normally can’t tell against an advanced player until after he’s started his forward swing.
Try adding last-second changes of direction to your game, especially when receiving. While it’s valuable at all times, it is when receiving that you’ll get the most bang for your buck by changing the direction at the last second. For example, if you are pushing aim to the backhand, and then, at the last second, as the server is reacting to it already, change and quick push to the forehand. Or do the reverse, aim to the forehand, and as he reacts to that, go quick to the backhand. You’ll be surprised at how often you can mess up the server in this way. At first he’ll likely get caught going the wrong way. Even when he adjusts, he’ll be rushed as he’ll have to wait longer to see where you are going.
You can do the same against sidespin-topspin serves, aiming one way, then going the other. It’s one of the best ways to force an opponent to use his weaker side. Just aim to his strong side, then go to the weak side!