Tennis is an impromptu exercise. You never know what's going to happen because the dynamic of the match changes with each opponent. It's also hard to predict because your opponent will make an adjustment to every adjustment you make.
Yes, there are general rules and guidelines, and yes, there may be general objectives you're trying to achieve strategically. But much like impromptu public speaking, where you wouldn't worry about exact words or phrases, don't worry about an exact shot or situation during a point.
Know the end goal of what you're trying to accomplish, and like a public speaker who doesn't sound scripted and speaks from the heart, let your energy flow and your natural instincts guide you.