A player that I looked up to (he's like a 5.5 now) tried to teach me many years ago that to settle his nerves he would only focus on one thing: looking at the tennis ball. He tried to make the only thought in his head to be staring at that ball. He even made it a little game to see if he could pick up some of the lettering.
I think I tried it but didn't really understand at the time the mental strength that goes into that. It felt like a cliche that a lot of people said. I didn't really get it until I gained a lot more experience. The mental effort required to think of only staring at a tennis ball is difficult for just one game let alone a whole match. That tip, in the end, served me well in many, many matches. It doesn't have to be just staring at the ball either, I often use my breathing as the one thing I'm thinking about and it works great. I make sure I'm taking good breaths during the point and exhaling during my shot.
I know The Inner Game talks about self 1 and self 2 or conscious vs subconscious. Let your subconscious play tennis and you will be in the zone (not thinking at all), the closest you can get to that is just by thinking of one thing.