Epictetus says this several times, referring of course to our struggle to live virtuously, in control of our judgments and desires and choices. You can be undefeated in this arena because these things, and these things alone, are in your hands.
You can be invincible if you do not enter any contest in
which victory is not up to you. [ Encheiridion 19 ]
Perhaps, but isn’t our experience that externals are sometimes much easier to master than inner virtues? Aren’t fitness and money and respect easier to claim than a reliable self-discipline & self-control, much less tranquility? It seems to me that I at least have a poor record in the contests for virtue I’ve entered. Maybe I’m 18-30-10, not exactly a contender. My record with externals is quite a bit better.
You can say of any new fighter that he can go undefeated, but name me a champion who did. In the struggle to master some virtues I do not see that victory is “up to us” in any meaningful sense. Will I become courageous enough to face the terrible things I will experience on the battlefield? Maybe and maybe not. If you begin to think "I am courageous enough to face anything ", the world will test you and you will probably not like the result.