Fighters that had a great fight, followed by one of their worst?

You could include Joe Frazier's once in a lifetime performance and victory against Muhammad Ali in the FOTC followed by Foreman's demolition of him in 1973. (The Terry Daniels and Ron Stander fights in 72' were more or less easy paydays against sparring partner quality opposition.)

The stark contrast between the version of Frazier in March 71' vs the version of Frazier in January 73' shows how rapid Joe's decline was in just 2 short years!

 

The fact that he almost beat Hagler is still so unreal to me. A lightweight leading on the cards by round 13 against the Marvelous One is a testament to how great Duran was.

 

Its the opposite but Langford v Fulton and Anderson is a fairly stark contrast.

And yeah theres Foreman and Buster Douglas. Liston is another obvious one. Joshua following up the Wallin and Ngannou wins with the Dubois fight is a recent one.

Think you can come up with a lot of examples with time. Things going great for a boxer until they fly too close to the sun is a common theme in this sport.

 

Joshua had a great performance when he cleaned up Fury's mess knocking Ngannou out and then got destroyed by Dubois in one of his worst fights ever looking like an amateur.

 

Tim Witherspoon beating Bruno and then losing to Bonecrusher maybe?