Saikyou Otoko to Kakutouou

Synopsis

He's the strongest man alive, yet he can't defeat the King of Fighting, who is apparently a shy little bookworm girl. Takeo's got it bad, until he comes up with a plan to finally win. It doesn't go as he plans, though, since the "King of Fighting" is truthfully a masochist, who loves to read about bondage and wants to become a sex slave. (Source: Pururin)