"Watch the Ball"
This often used tennis phrase sounds so simple, but it's really much less simple that most people commonly understand. If you've taken a lesson from me, not only do I say watch the ball, but I will often say: "Watch the ball and then keep your eyes on the point of contact with your strings and don't look up for a split second."
Why do I say that?
This is a much deeper explanation on how we must train our eyes to focus on right things, which will help you improve your timing as well. From Coach Edgar Giffenig:
I myself did not notice that I somehow did not watch the ball as closely on my backhand side as my forehand side; as a result, I have more frequent mishits on the backhand side. Do you have a side of your groundstokes where you notice more frequent offcenter mis-hits?
I recommend that all my students watch this video to get a deeper understanding of the concept of "watching the ball". - Coach Jason