A big part of being a successful tennis player is the ability to use knowledge and thoughts you have accumulated from experience, on the court.
There is usually a very small physical difference between most of the pros on the tour (and probably many of your opponents). This often makes the mental aspect of the game the variable that decides the winner of most of the matches.
Many of the best players in the world tend to play by instinct. They do this by reminding themselves about a basic technique idea just prior to the start of the point (watch the ball all the way through the point of contact), followed by playing the shots that are most natural for them based on the opponent's shot, and finally reminding themselves in between points what their basic plan is (play aggressively from the baseline and look for a shorter or weaker ball to attack).
There is very little thought process happening during the actual point, especially in the modern game, which has gotten incredibly fast. Points are short, the time in between hits is short, and instinct and reaction tend to be the best way to handle these situations. Players can think in more detail on the changeovers and analyze what is going on a bit more, so that they can make adjustments to their game if necessary.
In short, try to keep your thoughts minimal and simple when you are playing a point and your results will be much better in your matches, especially under pressure.