Your opponent can be anything. If you're an athlete, it could obviously be the individual or team you're competing against, a teammate you're trying to beat for a spot, or simply yourself. If you're a student, it could be an exam, a project, or a presentation. If you're a lifter, it's the weights. If you're a teacher, it might be your students' attitudes or the expectations for the class set by your superiors. If you're a human, it could be the day in general and how it's been going. Regardless of your field, you have an opponent.
You absolutely know what you're capable of, and when you know that defeating your opponent requires a 70% effort, what do the majority of people do? They put forth a freakin 70% effort. And that's exactly how the greats lose to the average. If there's any fluctuation, if you drop just a hair below 70% or your opponent happens to bring their best, you're walking away defeated. Not just defeated, but embarrassed. You should've won, but you didn't deserve it. You were more than capable of winning. Everyone expected you to win. You have no excuse.
If it requires 70%, give 100%. If the guy next to you is slacking, set the tone with your excellence, embarrass him, and make him think twice about cruise-controlling. If your students have bad attitudes, be that leader. If the test is supposed to be easy, blow it out of the water. If the weight is relatively light, perfect your form and nail the lift. Destroy the standard. Throw off the curve. Make it clear that your opponent should never have even considered competing against you, and make dang sure they never want to again.
Sinking to your opponent's level is a dangerous waste of your time. Don't mess around with that. Bring your best, do your best, be your best always. Excellence in everything.