Almost every player has been told by their coach or tennis professional to "watch the ball". This sounds like such a basic and easy instruction to follow, yet many of us still struggle to execute it.
One of the problems is that seeing the ball actually make contact with the racquet is not likely, if not physically impossible. The ball is traveling much faster these days and so is the racquet. My advice is to try to watch your opponent's point of contact, follow the ball with your eyes all the way until your racquet makes contact with the ball, and keep your eyes right at the impact point after the hit.
Try using your peripheral vision to follow the ball off of your racquet (to determine if it is going in the desired direction) and then immediately recover and repeat the process. It is also a good idea to look for something specific on the ball (the writing, color of the writing, or the spinning of the seams) so that your focus is more specific than just watching the ball come towards you.
Remind yourself of these ideas before each point starts and you will likely have more solid hits and a better reaction to the ball that is being hit to your side of the court.