In most cases before playing a match you know something about your opponent. You either know who they are, and have played them before, or you have some clues as to their calibre. Even if you have none of these things, you can study them during the warm-up. What looks best and worst about their game?
As much as possible, you should study your opponent as they play other matches. Look at their technique. Refer to my ‘Things not to do’ post to try and identify problems with their technique. These flaws can then be exploited.
For instance, if they play with very straight legs, you know their movement is going to be poor. You should test their movement back/forward and side to side. They will also have a very hard time with high looping balls over their shoulder height since they wont have spring in their legs to help them hit through the shot. Volleying at the net vs. someone who has stiff legs is good because with stiff legs it takes them longer to recover after each shot.
If they have a very western grip and like hitting the ball with lots of topspin, then test whether they are capable of returning your low slices.
If their grip is continental, then hit with heavy topspin, and they will struggle to no end.
If they keep their arms too straight on their backhand, they will struggle with high balls to that side.
If they have a big looping swing instead of a compact one, then they will require perfect timing to connect with the ball. Try mixing up the pace of your shots during the rally – hard ones, loopers, slice, etc.
If they are a serve and volleyer, stand in closer on your returns to rob them of time to get to the net. They’ll have to hit their first volley from far back in the court.
If they’re uncomfortable and bad around the net, then consider hitting short slices to lure them in.
If they’re out of shape, go for long rallies hitting neutral shots down the middle to tire them out, and if you feel comfortable with your shots, get them running side to side using wide angles.
The possibilities for exploiting poor technique are almost endless. But generally you can trust that if someone just looks to be technically poor, that they will generate lots of errors.