As in 'which sports involve skills which are transferable to boxing?' Shot putting and fencing have some small crossover, i guess.
What about track and field? Track and fielders have great upper and lower body strength and endurance.
Great call. I would have said the Medieval Joust was pretty close too. Like, whoever had the best horse, best armourer, bestest King supporting him always won. Bargaining Power, Yo. The Chivalric Community had it.