Racquetball is different than other sports in that you respect a much larger safe area for your opponent. If your opponent is near the ball when you're about to swing, simply stop and play it over. It's not like volleyball or basket ball where you can reach next to or around your opponent or partner and hit the ball when it's close to them.
Before you hit the ball, always know 3 things: where your opponent is, where the ball is and where you are. If you don't know where your opponent is, you might hit him with your racquet with your backswing or downswing. If you can't see him, just stop playing and say "sorry, but I wasn't sure where you were, and didn't want to swing".
The safety rule is: don't swing based on where you think your opponent is, or where he should be, but instead visually verify that he's really out the the way.
A situation where this comes into play is when you're chasing the ball around two or three walls in back court, and while whirling around, you have lost sight of your opponent, who may have gotten trapped against a wall and is now in your line of fire. If you don't have time to look for him because you might lose track of the ball, just stop and explain that you lost track of where he was and that you didn't want to endanger him.
Of course as time goes on and you become more experienced, you will know where everyone is almost 100% of the time.
If your opponent is standing in your way a lot of the time (which he is not allowed to do), don't hit him with the ball to teach him a lesson about court positioning. Instead, print out this page on how to stay of the way or give him the link.
If you're interested in practicing hitting the ball really hard during a game, the safest shots for hitting the ball hard are the splat and when everybody else in the court is standing behind you.
Always ask your opponents to wear safety glasses and pay attention to that inner voice that is telling you that there might be someone standing a bit too close to your swing.
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