This simple but brutal contest is another good one for DF.
Players often ask me about getting into bar fights, pit fights, etc. But honestly, it never works out. As long they are winning they are fine with it, but as soon as they start to lose they'll turn it into a lethal battle. Since most of my games have lethal battles anyway, it's just all downside - we get just another combat, and nasty consequences like dead NPCs and Social Stigma (Criminal Record).
Better is something that just doesn't act like a fight but uses fight rules. Something that is just stupid.
Two opponents face one another at arm's length. They take turns delivering punches to the other's face. Defending is bad form. Make a Will roll not to flinch - flinching allows a free penalty shot by the opponent. The loser is the first one to fall down. Keep your cleric handy for afterward!
Traditionally opponents take All-Out Attack (Strong) and Telegraphic Attack for a wind-up full power strike. Alternatively use Shoves, instead, and just make the contest about knocking the opponent back.
Options include the Chessboxing approach, but with drinks (one round punching, one round of shots), or having the fighters precariously balanced (DX roll not to fall down when struck, DX roll not to fall down when striking!), or other mixed variations.