When you start your warm up, you first need to get used to the conditions and your feel and form for that day. It's not realistic to expect that you'll be feeling superbly well every day.
Check the conditions: Is it windy, sunny, hot? And then adapt your shots and tactics accordingly. Check your feel for the ball, racket and your general mood, and ways to improve on them.
Once the match has begun, you'll need to adjust to your opponent's play. Most of the players don't play the same in the warm up as they do in the match, so you are very likely to see and feel new speeds and spins of the ball.
Don't panic if you can't adapt in the first minute. Your brain is getting the information, unless you're too emotional. Remember previous matches, and how you found your timing on returns or volleys or whatever caused problems at first.
Just watch the ball well and notice what is happening: are you late, too far, is the ball too high? Then look for ways to gradually adapt. Don't force it; your game will adapt if you are relaxed.
The most important shots that define how the point will be played are the serve and the return. If you have a weak serve and return, then you'll be on defense the whole point.
A good serving tip is to first find your serve. Don't blast your first serves in the match at your full power. Start at 75 percent, then increase the speed and locate the best feeling and percentage of shots.
The same rule applies to returns: Start with medium-paced returns down the middle and then add speed and placement when the match progresses. The main goal of the return on the first serve is to get it back deep down the middle. On the second serve, try to make your opponent run, so a cross-court return would be preferable.
The game is based on your groundstrokes: forehands and backhands. You need to find them also when you start the match. Start with long cross courts 2-5 feet over the net and aim 5 feet from both side and baseline.
When you find good length, start utilizing short cross-court and down-the-lines shots to make your opponent run.
The same principle applies to volleys and overheads. It may take you 10 or 20 minutes before you play your first volley or overhead, so don't expect a perfect shot. Or maybe you can expect it, but don't get too upset if it doesn't happen.
Actually, if you often play at the net you're maybe aware of this fact: You need to develop the feel and timing for your volleys. You might play a couple of poor volleys or overheads first, but don't let that discourage you.
You're showing your opponent that you are not afraid to come to the net and you are adapting to his shots. Imagine how good it will feel when you hit excellent volleys and overheads towards the end of the set.