The good news is that concentration can be trained, it is not an innate skill, it can be improved and constantly developed. However, what are the ways to work on this concentration during the match? There are a lot of them, not everyone works at all, but it is certain that everyone should try what suits them and choose the most suitable method for themselves.
One way is to focus attention on a specific thing, e.g. footwork (where we try to feel their dynamics, feel every muscle and every step), on the ball (we focus on its movement, on every detail, on its color, hair, etc. .), we can also focus our attention on our breath (this allows us to return to the "here and now" and calm our nerves a bit). A good way is also to create a ritual for yourself, which you will systematically repeat in situations when you feel that your thoughts are drifting away or when you just want to be "in the game" all the time. It can be stretching the strings in a rocket, flicking the flour off your tennis shoes, jumping up, an invented slogan that reminds you of concentration, or anything else simple to do between the breaks in the game. After each point you can also say the result aloud, and during the match listen to the sound of the ball or rhythmically count the plays, e.g. when you hit the ball you say "one" in thoughts, and when your opponent does it, say "two".
What should you not do? Certainly internal or spoken discussions with yourself will not help you calm down and enter the match. Yes, it is sometimes good to shout at each other or say encouragement, but the more you get into the analysis of previous points and start talking to yourself, the harder it is to focus on the game. So if you find yourself starting to talk to each other, know that something is wrong and instead of continuing the fierce exchange of opinions, do something from the examples given above to focus your attention on something else.