Your brother-in-law is just an egotist. Get kick him in the balls and strike him down while he's weak. Seriously, mentally you may think you can continue but your body's condition will beg to differ. That is how professional fighters (boxers, UFC fighters, etc.) end up in a life or death condition. They continue to fight even though they cannot handle much more and neither the ref or coaches do anything to stop the match.
I can put up a good fight against people who I am evenly matched against, but may have to also use intelligence to overcome someone who is stronger than me. Chances are I would have the upper hand against an even opponent unless they too have boxing experience and can do martial arts. I have agility, stamina, and coordination, but can lose focus due to occasional brain farts that result in a lack of registering what I am seeing, if you know what I am trying to say.