Emotions are a part of tennis. Players tend to feel miserable when they lose and great when they win. This is true during the match as well. A winning point can give you an exhilarating feeling and a losing point can tear your insides apart.
For almost all players, it is better not to show your negative emotions. John McEnroe was one of the few players who I felt, played better after getting irritated on the court. Most of the time that negative energy carries into the next point which results in several points, and even games, being lost in succession. Positive emotions can propel you to heights you never knew existed. However, it is important to channel those positive emotions correctly.
If you are a high-emotions type player, a fist pump or a "let's go" can be used quite a bit when something good happens. But it’s sometimes better to use the outward positive display more carefully so that it still has the intended result as the match goes on. I’ll use Pete Sampras and Roger Federer as my examples:
Both of these players use the fist pump, or a "yes!", but only a few times a match. They both have a tendency to stay measured with their emotions and I believe this helps when it gets to the crucial stages of a match. Rafael Nadal tends to be much more animated since he is that high-energy type of player. Novak Djokovic is somewhere in between these players.
Try to figure out who you are as far as your emotions are concerned, and adjust how you display your feelings accordingly. My advice is to stay on the lower end of showing how you feel (unless you are very high energy and it helps you play better), and use your fist pump or words of encouragement at an appropriate time. Try not to have negative outbursts or reactions. This will help you stay calm, cool, and collected at crunch time and allow you to perform at a high level even if there are some nerves involved.
Your ability to channel these emotions and feelings will be a big help when you are trying to win the match.