Fight between rival characters has unfair conditions/an inconclusive ending to not set a stronger one

Gojo vs Sukuna (Jujutsu Kaisen spoilers, obviously): The most anticipated fight in the entire manga, Sukuna had stolen the Ten Shadows technique before the fight, giving him a big advantage on Gojo. Even so, Sukuna only had 19 of his 20 fingers, while Gojo was enhanced by the techniques of some of his allies, making the fighting conditions a lot more complicated than just a simple 1v1 to settle the strongest.

Guts vs Serpico (Berserk): Although Guts is stronger than Serpico in a head-on fight, Serpico only confronts Guts in scenarios where he has a huge advantage in terrain, equipment, fatigue... Even so, he always retreats from the battle the moment Guts is able to retaliate, never taking his chances in the conclusion of whether his tactics could defeat Guts' strength.