Eddie Hearn, promoter of Anthony Joshua, has personally seen a re-lit fire in the mental frame of the British star.
Back in June, Joshua seemed mentally exhausted when he dropped four times and stopped in the seventh round by Andy Ruiz at Madison Square Garden in New York City.
The rematch is on Saturday night in Saudi Arabia, as Joshua aims to reclaim the WBO, WBA, IBF, IBO heavyweight titles.
Hearn believes the contest is a "must win" for his boxer.
Another defeat, to a boxer who in the first bout was tagged as a massive underdog, would be a huge hit to Joshua's career. He would certainly lose his place among the elite in his division, and a rebuilding process would have to begin.
"I loved it. He wants this bad. His back is against the wall. He has prepared better than he's ever prepared because he's fighting a guy who beat him, and knocked him down four times at Madison Square Garden in front of the world. He wants to claim back what is rightfully his," Hearn told Sky Sports.
"There is no warm-up fight, no getting back into the swing of things. He said: 'I want this fight now'. It is very dangerous, fighting a confident guy who will think: 'I must get into range and let my hands go against Anthony'.
"Anthony has to be disciplined but also hurt Ruiz Jr. This is one of the biggest heavyweight fights of all year, the biggest fight of the year and the decade. The biggest heavyweight fight of the century. I'm nervous and excited, but pumped after seeing AJ because he is pumped. I haven't seen him like this for a long time. He is ready, he is hungry. I see that bad kid again, and I love it."