I’m not talking about just a guy who seemed to be good or pretty good defeated a great (i.e. Buster Douglas vs. Tyson), but more where a fighter showed a major evolution in growth in his approach to the sport. The best example I can think of is Roberto Duran in his third fight with Esteban De Jesus — up to that point he was a savage, swarming pressure fighter with good inside skills and power and a great body attack, but hadn’t shown the measure of ATG skill we came to know him for at mid-range and with his feints and generalship. De Jesus had handed Duran his first defeat and lost the rematch, knocking Roberto Duran down in the first round both times. Freddie Brown, I believe, told Duran if he rushed out against Esteban in the third fight he’d be waiting with that left hook and knock him down again. So Duran transformed himself, showing those boxing skills to set up his attack and dominate and batter his foe. On this night, Duran took the quantum leap forward to being the all-round fighter he was to become. What are some other examples?
Julio Cesar Chavez moving up in weight to challenge the much feared and powerful lightweight champion, Edwin Rosario. Everyone knew Chavez was good beforehand but no one including heavyweight champion, Mike Tyson, who was working in Rosario's corner that night knew just how good he really was. You could say that was the breakout fight that Chavez truly earned his nickname of JC Superstar.
Roy Jones Jr utterly schooling Toney. Sure Jones had got to P4P #3 against Toney's #2 but that was more on promise and perception than anything with any depth. Toney had a lot more experience and some really good victories behind him yet he got absolutely whupped. Jones put it all together.
Azumah Nelson vs Salvador Sanchez is another. Nelson of very little experience and resume rises to the occasion and doesn't get beaten for 8 years. IMO Hearns, despite some dominant showings against decent fighters ramped it up another level vs Ceuvas. I would argue George Foreman went to another level when he smashed Frazier, *Fraizer sorry. Mike Tyson i believe put it all together against Berbick. Mike Weaver's turning point was the fight with Holmes. He had found some dedication and confidence and fought well. There's a few to get things going.
Maskaev both times against Rahman, particularly in the rematch when he won the WBC belt. His chin never was very sturdy against lesser fighters, but against Rock he showed suprising durabilty and toughness in both fights.
Buddy McGirt against Simon Brown. Before that, McGirt was seen as one of those fighters who might win the odd alphabet belt if they stuck around long enough, but would never be a divisional kingpin or amongst the elite. It's not that he won - it's how he won. Dominated from start to finish and made the (then) best Welterweight in the world look a mug. Came as a surprise to a lot of people.
Lloyd Honeyghan. Yes Curry had excuses but Honeyghan confidence for this fight and the way he lifted was incredible.
Brandon Rios showed a new level of potential in his fight with Anthony Peterson and then demonstrated an extra gear on top of that against Miguel Accosta I thought Oscar De La Hoya raised his game when he punched Paez's lights out. It was definitely a step up from getting dropped by Narciscco Valenzuela or struggling with the southpaw stance of Jimmi Bredahl.