A sense of inadequacy often compels us to ask ourselves "who am I?" This is as true when it comes to tennis as it is with "real" life. When faced with a tough or unknown opponent we are often forced into a corner where we have to question our identity. This is particularly true at the higher levels of the game where opponents are not easily labeled into neat and manageable categories. For example, it may be tempting to generalize a player like Nadal as a baseliner but give him a mid-court shot and watch him follow the approach to the net for a put-away volley. Similarly, players like Isner and Karlovic can defend like the best of 'em.
Therefore, when facing an opponent without glaring weaknesses or weapons, it is often advisable to focus your analytical energies inwards. Don't worry so much about who they are; try to figure out who YOU are. How do you like to win points? What kind of points make you comfortable? Close your eyes and go to your "happy place" where you're playing your best tennis. What types of shots are you hitting? Do you see yourself scraping more? Putting volleys away? Blasting shots from inside the baseline? Etc.
Once you figure out who you are as a player, imagine your game as a tree. The best game that you can possibly play is the tree trunk. The various branches are off-shoots of your main game... slight variations on your various game. Imagine, for example, that you're an attacking player who is facing a solid opponent. You try your "main game" - perhaps serve and volleying - but it doesn't pay the dividends that you desire. Does this mean that you are going to discard your "game style" and completely switch tracks? That may be a good play once in a while. However, a better play would surely be to figure out all the other ways in which you can continue your attack. Maybe a straight serve-and-volley play is not effective because the adversary nails her returns at the service line. Great! How about serving, stepping into the baseline as if you're going to come in, then taking the short ball from the opponent with an approach shot and following THAT ball to the net (3-shot play)? Can you imagine other ways in which you can attack? How about one of Henri Leconte's favorite plays which was to hit a drop-shot and follow the ball and punch the volley past the opponent to just managed to scramble to the ball? Or how about hitting a heavy topspin lob and sneaking in when the opponent backs up to the fence (the "ten and under play")? Alternatively, work the opponent around 5-6 shots before you capitalize on the opening (don't get discouraged just because the routine 2-3 shot combination is not working)? So the lesson is: 1. figure out who you are and what you like to do with the ball; and 2. imagine all the slight variations on the game that you can implement against this particular opponent. Rather than thinking in terms of "changing your game", think "refining" or making "slight adjustments" to fit the particular situation. You will be more effective when sticking to the proximity of your bread-and-butter play than trying a game style that is diametrically opposed to your comfort level.