One of the fundamental rules of percentage tennis is to avoid changing the direction of the ball when playing defense. In other words, if you are under attack and the ball comes to you crosscourt, the higher-percentage play is to return that shot back the way it came.
Why? Whenever you change the direction of the ball you increase the number of variables that you have to account for in order to make a good shot, and more variables means there are more things that can go wrong, which is the last thing you need when you’re in trouble. Changing the direction of the ball requires your racquet to travel in a path that’s slightly different from that of the oncoming ball, and your timing has to be more precise to pull it off. But if you return the ball in the same direction your swing path will be more in line with the approaching ball and you improve your odds of making good contact. Look at it this way. If two cars are traveling on paths that are perpendicular to each other they are less likely to crash than they would be if they were speeding toward each other head on.
So when should you change the direction of the ball? Wait until you have either a short ball or a ball in a neutral rally that affords you the time to set up properly.