OK. So now you have prepared properly for your match and are ready for all possibilities. You have played pretty well in a close match and you are a break up and serving for the match. You’re nervous. That’s normal. It’s not something to be afraid of, but you do need to recognize what is happening to you physically. Sometimes you feel like jelly, other times you feel your body turns to stone; hence petrified. Anyhow, you have to try to relieve whatever symptoms you have and play through them. If there is tension in the body, obviously the racket does not swing freely and generate pace. The body stiffens up and the arm can’t accelerate through the ball.
I think the best solution is to make sure you take your time, take some deep breaths, and swing the racket through in a service motion without the ball a couple of times. Try to maintain the sensation of reaching up, going after the ball. Also mime the tossing action as that arm tightens up as well, and be decisive about where you want to serve.
Most importantly, keep your feet moving during the points so the nervousness doesn’t get them rooted to the ground and you wind up resembling one of the statues that surround the Foro Italico.