The tennis court can be a very lonely place for a singles player. In between points it is easy for our mind to wander and be distracted. People wandering around the behind court, planes flying over, other matches going on around you, all have the potential to take your focus away from the match.
It is impossible, nor is it recommended that you stay focused during the whole match. You need to have many breaks, in between points. The challenge, is to regain a high level of concentration quickly at the start of each point.
A technique, I am sure that you have seen the professionals do, is to maintain straight strings in your racquet. Keeping your strings straight makes very little physical difference, however is a great concentration technique.
You will see top players continually straightening their strings before each point. While they are straightening their strings their concentration is very focused. It gives them place for their eyes to focus while they think about the next point and how they going to execute their plan.
If you stick to this technique is easy to see when you are not concentrating. If you look down at your racquet, notice that all your strings are not straight, you need to concentrate more and take more time before points.
Try it out and let me know how you go!