So, you want to react to an opponent's shot like the pro's? That's simple - as long as you follow the three principles to fast reactions. They are:
- Ready position. If you have a good ready position to start from, it's much easier to react quickly. Many players have poor ready positions, and so aren't ready to move right away. You need to recover quickly from the previous shot (very important, often a problem!), stay balanced, weight on front inside part of your food, feet at least shoulder width apart (as far apart as you are comfortable - watch the pro's, but remember they do physical training that allows their very wide stances), with racket pointing right at the expected contact point of the opponent. Never wait to see if you have to move - expect to move.
- React, don’t guess. Many players feel panicky and so try to react too soon, and so their first move isn't the right move. Take your time and make sure your first move is the right move - don't try to guess. (There are a few exceptions to this, such as if the opponent is predictable or if he's about to put the ball away and you have to guess where it's going.) Always remember - You have more time than you think! It's surprising but true that consistent quickness comes from taking your time.
- Move to the ball. In a fast rally, many, probably most players react first by reaching for the ball. NO!!! Always react first by reflexively moving your feet. (There are times where you are caught out of position and are forced to reach - but only do this while also stepping.) The key thing is that reacting by moving your feet is a habit you can develop with practice. It needs to become your first instinct. One way of developing this habit is to focus on balance - if you keep stay balanced, you are forced to move rather than lunging at the ball.
And that's all there is to it. Did I mention you have to practice to develop these things?