Im Kampf zwischen dir und der Welt sekundiere der Welt.
In the struggle between yourself and the world second the world. [Kaiser/Wilkins]
In the struggle between yourself and the world, hold the world's coat. [Hofmann]
Kaiser/Wilkins marks this one cancelled, while Hofmann does not. Huh?
The superficial and not particularly interesting meaning is obvious enough: the world is more powerful than you are, so a fight with the world is one you're bound to lose.
In a duel, however, the second's task is to bear witness; he is there to make sure the fight is conducted fairly. With that in mind, the aphorism would mean instead that, in your struggle with the world, it's your own cheating, not the world's, that you have to watch for.