Of course you have heard every tennis pro say "Watch the ball" and of course this is true.
However, how many of us, after watching the ball, continue to watch the ball go over the net after making our own line call on it? Remember, you call balls in and out on your side of the court and your opponent calls balls in or out on their side of the court. So, what should you do after watching the ball and hitting it? You should assume all shots, especially the ones that you hit, are "in". You are wasting your time making line calls on your opponent's side of the court. Look at the closest opponent's racket face. For example, your opponent is on the baseline and you see the racket draw back in a traditional ground strike to drive the ball, or you see the racket head drop down getting ready to hit a lob or you see the racket heard setting up high for an under spin shot. You might see that your opponent is in an awkward position, putting them in the defensive while you assume the offensive and vice verse.
When your opponent is at the net, if you can see their racket set up high (above the net level) then they are on the offensive. If you see their racket low (below the net level) they will be more often in the defense. When you see them standing still with their racket cocked high and behind their head they will be in offense.
Regardless, you waste time by watching your ball land on your opponent's side of the court. Take that time to gain more information of what is coming next. It is good to "not watch" the ball all the time. Keep watching until you hit it, and then focus on your opponent's position and racket.
Good luck on the courts!