Shannon Briggs says he's heartbroken Wladimir Klitschko hung up his gloves ... 'cause he never got the chance to KICK THE LIVING HELL OUT OF HIM.
Briggs famously tormented Klitschko for years ... popping up on him in public places and trying to get Wlad to meet him in the ring (the paddle boarding ambush is still classic).
Klitschko just announced he's retiring, and when TMZ Sports talked with Briggs he told us he wishes the all-time great well -- that doesn't change the fact that he would've KNOCKED HIM OUT.
"I would've knocked him out cold! I'm sad, I'm sad he's gone, champ!"
Some things never change.