His downward slide in the boxing ring was pretty sharp. He was one hard mofo but I think he took a bit to much punishment, it caught up in the end. 2000-2004 he was a force to be reckoned with, absolute beast. But his last three fights he did not look like the same fighter IMO.
And Michael Brodies, his 2 fights with Brodie beat every last bit of his boxing career out of him, Chi was a very good fighter.
He retired from boxing. Boxing is no longer popular in Korea, and he wasn't making much money doing it. Money's much better in K-1.