If you have the skills to be offensive, for example if you play someone much weaker than you, then be offensive. Offense is always a better choice. It dictates control and rhythm, defending does not.
Obviously, playing offensively is different from playing foolishly. If your opponent has a good serve and you're not able return it consistently, then play defensively when returning serves.
Many mistake "offense" as hitting as hard as they possibly can. That's wrong. Playing offensively simply means that every shot you make is with the intention of making your opponent defend.
Many also have problems with the so called "pushers" because they're not being able to put these "pushers" on the defense. Hitting the ball as hard as you can but having that ball land short and moving toward your opponent is NOT an offensive shot for goodness sake.
If your opponent is far back at the baseline rallying back and forth with you and you all of a sudden pull a drop-shot to make him run to it and possibly get out of position is an offensive shot. If your opponent is at the net and you're blasting balls straight at him and he's getting them all back, then that's not an offensive shot, lobbing the ball deep and making him chase it and give a weak reply IS an offensive shot.
As a general rule of thumb:
Offense = forcing your opponent to defend.
Offense = forcing a weak reply because your opponent is out of position.
Offense = setting up for another offensive shot.
Offense = you're dictating the flow of the exchange, you're doing something so that your opponent only has the option of putting the ball back into play and not much else.
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