Like NuclearNacho said, this is pretty good for a free online boxing game.
My tactic is to punch my stamina out, then try running away (This actually works some time), and then punching again when my stamina is full or the opponent is close.
While being knocked down by any punch is realistic, I think that shouldn't have been implemented to such unrealistic boxing game.
I became champion after 6 or 7 wins and 5 losses. I needed more stamina for that.
I think you should have made more different punches. For example, hooks could be shorter but do more damage, and power punches would only hit if your opponent doesn't hit you first.
And since the game is good, why not more opponents?
I can't lower my stars to 4,5. (I gave the stars months ago.)
I'd love a sequel, but I guess after years that will never happen.