You guys are all wrong.
His decline started when he divorced his wife Oksana in 2006. The marriage kept him on a steady and disciplined regiment. He was able to last for a few years after their divorce, but then he got tied up with Marina (Ms. Damsel) in 2009 and a new group of flunkies along the way (religious fanatics, Van Damme, the tall sleazy guy that was always celebrating with him).
Once he became more famous, he lost the hunger to win. He came to fight looking flabby and depressed, like he hadn't been training. When he started losing, he knew he messed up. That's why he re-married Oksana; she is probably the reason why he continues to fight.
Win or lose...Oksana is the real GOAT.