In terms of confidence, there are two terrible things you could do going into a match. The first thing is to go into it thinking you are going to lose and the second is to take your opponent for granted by thinking he/she is not any good.
I wouldn’t quite recommend adhering to the Middle Way of the Buddha Sidhartha Gautama in terms of confidence, but it is definitely best to avoid being at either extreme before a match. I think it is better to come into a match with unwavering confidence that you are able to win the match instead of being directly in the middle and not having any idea or thinking that you have no chance of winning or losing.
If you enter a match believing that you have no chance of losing, chances are you are going to find yourself surprised at some point in the match. Either the other player is going to get off to a hot start and you will find yourself down a break or you will be up early and allow them to sneak back into the match. In any event, it will cause you to perhaps play erratically and lose sense of any semblance of a strategy you have and force you to gather yourself and get your act together…otherwise you will be in a big hole and drop a set or even worse, lose the match.
It is imperative to feel confident in your abilities and also take note of your opponent’s skills and tendencies during the warm-up period if you haven’t yet seen them play.
Regardless of what you see in this period, your confidence shouldn’t change but your preparation and strategy might…as you pick up on things in the other player’s game that you should take advantage of. Warming-up is just as much for your own skills as it is diagnosing your opponent’s skills. Maybe they seem to have a weak backhand, have shotty volleying technique or have trouble with slice. No matter what you see, you should remain confident that you have the ability to expose the player(s) on the other side of the court and be able to play the best match you can against them.
In terms of your own skills during the warm-up…do not get rattled. It shouldn’t matter if you think that you aren’t hitting or serving well before the match. Furthermore, do not find yourself in awe of the other player. They could look like Pete Sampras in warm-ups and end up being more like Tommy Haas (that might be an on-going joke for my writings). Some people simply are practice warriors that look and play unbelievable until the first serve is hit. Just as you shouldn’t worry about your failures in warm-ups, you shouldn’t let the success of your opponent get to you either. It takes two to tango and you should be confident that you can lead the dance regardless of how the warm-ups go.
In conclusion, the only thing that should really change before or during your match should be your strategy. You should always consider different approaches and remain pragmatic and be able to improvise based on what you see…just do not change how you feel. All that can do is put a self-imposed handicap on the match and start an up-hill battle or a slide down a slippery slope. Stay unwavering on the plateau.