Again, Master Sun Tzu tells us
"If you know the enemy and know yourself you need not fear the results of a hundred battles."
In battle, as in tennis is important to know and fully understand your opponent's likes, dislikes and motivations. If you do, you can tailor your strategy to defeating him. Evaluate your opponent from the ground up and in both a technical and tactical sense.
For example, is the opponent fast (i.e., does he cover long distances quickly)? Is the opponent quick (i.e., as opposed to being fast, a quick player is great ad adjusting to shots struck with pace right at him)? Is the opponent strong (i.e., can he punish sitters)? Is your opponent smart (i.e., is he the type who plays the same points over and over or does he have several cards up his sleeve)? Does your opponent have endurance (i.e., will he be able to go the distance or is he looking for a quick kill)? Is your opponent aggressive or passive? Does he wait for returns with a forehand grip, backhand or neutral? Is the opponent protecting her backhand? Is the opponent comfortable at the net or hitting overheads? Is the opponent a front-runner or a snake in the grass (i.e., does she play better when she's ahead or is she the type to wait until you break your concentration to topple you? How does your opponent serve under pressure? Who are the type of players that have beaten her before (bashers, counterpunchers, pushers, serve-and-volleyers, dinkers, slicers)? How does your opponent handle pace, spin, height? Is the opponent quick out of the starting blocks or does it take her a while to get going? How is the opponent mental state - is she easy to rattle or ice-cold? Does he look like want to be on the court or is there another place he'd rather be at that particular point in time? What are his favorite "plays" or "patterns"? Does he prefer to serve at certain targets at big points? Does he take chances with the returns or does he play steady? Does he have any "hitches" or peculiarities in his shots which either prevent him from hitting certain balls or are likely to break down under pressure? Is he a fast-court player or does he prefer slower courts? Does he handle wind/sun/heat/altitude well?
In other words, evaluate your opponent so that you can formulate a game-plan in advance (if you're the "nervous type", thinking about strategy in advance will also help you take your mind off the player or the situation). If you've never played against this player before (and none of your friends have either), determine if you've played against a similar player in the past. Go through your mental "rolodex" of plays and figure out if this player resembles someone you've played against before.
If not, figure out if his forehand is like Player X's and Backhand like player Y's and draw a composite sketch of what you'd do against those players. Get a proper warm-up in advance so that your actual match warm-up is utilized to pick up on the subtle movements from your opponent which either indicate likes/dislikes or weapons/weaknesses and also "where" the opponent might go with that particular shot during the match (e.g. does he always go cross-court when he's hitting an open-stance forehand? If so, start leaning towards that spot early).