In the fast game of tennis, your instinct is a very important asset that should be used in your arsenal. Some may have it, some don't. Tennis is not jsut a ball-bashing sport, it's a tactical game. Here are a couple of good tips that you should apply next time you play tennis.
1. Sense danger - be aware that you just got yourself into defense. If you know you just hit a ball back that you know your opponent is going to attack on, bring your court position more back a couple of inches! That way you have enough time to defend the attacking ball.
2. If the story is the other way around, then if you know that the ball you just hit is going to trouble your opponent (let's say a deep ball into the open court) then bring your court position up more right onto the baseline to get ready for the weak response. That way you will take time away from your opponent - making him run more. And also you can shake up his feet and hit the ball behind him!