In my experience a lot of tennis coaches advise to not think while you are playing. Not really good advice in my opinion.
For one thing, it’s impossible to not think at all while you are playing. I’m not sure of the exact science but it seems like the mind has a thought every millisecond. Let’s just accept the fact that the mind is always at work, whether we like it or not.
What we are really trying to accomplish with ‘not thinking’ is focused or directed thoughts. For example, when playing a match you want to focus your mind a great deal on playing the ball, not your opponent. You might also want to observe what your opponent seems to be doing well and not well so you can capitalize on her or his mistakes. If you are just out for a practice, the mind should be directed to holding your aim points, concentrating on two or three things you want to work on during practice, and so on.
The key things to not be thinking about are points, scores and anything results oriented. Success happens when we trust the process. This is what is meant by staying in the moment.
So stop paying attention to those voices telling you to not think. It’s unrealistic and it’s not in our nature. Instead, re-frame the idea by reminding yourself to aim your thoughts in the right way.