Artur Beterbiev and Dmitry Bivol will finally step in the ring opposite each other on Saturday in Saudi Arabia.
Boxing fans have been waiting to see the light-heavyweight world champions fight for several years.
All four titles will be on the line in Riyadh, with 39-year-old Beterbiev holding the WBC, WBO and IBF belts and 33-year-old Bivol the WBA (Super) champion.
Both men are undefeated and the contest is being promoted as a potential classic in the making.
You can follow live text commentary of the main event and undercard, which includes two Britons in world title fights, from 19:00 BST on Saturday on the BBC Sport website and app.
Who will be the first undisputed light-heavyweight champion in the four-belt era? Who will suffer their first defeat? Could a draw be on the cards?
BBC Sport has asked around the world of boxing for predictions.
Retired world champion & boxing coach Anthony Crolla: “You can tell what a good fight it is because so many people are finding it hard to pick a winner, and I’m one of them. If I’m really pushed, I just don’t think Bivol will have the firepower to keep Beterbiev off. I see Beterbiev will wear him down and maybe get a stoppage around the ninth or 10th round.
“But I also wouldn’t be surprised if we see a very disciplined performance from Bivol where he outboxes Beterbiev, hit him and move him, similar to how he did against Saul ‘Canelo’ Alvarez.”
WBC lightweight champion Skye Nicolson: “It’s the typical brawler v boxer, and I have to back the boxer. I find it hard to see Bivol being caught in a situation he doesn’t want to be in. He’s so good at controlling the distance and pace of the fight.
“But if anyone can do it, it’s Beterbiev. My heart says Bivol.”
Retired world champion Barry Jones: “Beterbiev has to be a slight favourite. He’s knocked out punchers , he’s knocked out boxers and he can knock you out in the first round or the 12th. There is no round, second or minute when you’re really truly safe with him.
“But I will still go with Bivol. I just have a feeling that his intelligence, patience, discipline and calmness of approach might just be enough to ride the dangerous moments and get him through in a close decision.
“Bivol might have to get off the floor a couple of times but I think he could have an early enough lead to sneak home.”
British heavyweight champion Fabio Wardley: “I am edging toward Bivol but this is probably one of the hardest fights to predict in boxing at the moment. It’s a real pick ’em fight with two men with completely different styles. Beterbiev is a destroyer who can clean through everyone. Bivol has great power but is such a clean and precise boxer.
“It’s a question of who can enforce their style best on the night and make the impact.”
Heavyweight Frazer Clarke: “I keep changing my mind. I’ve been saying Beterbiev the whole time but I now think Bivol could take this one. It’s a world class 50/50, though.”
Retired world champion Darren Barker: “I would have Beterbiev as the favourite. When you have that power, he knocks people down with jabs. How can you not have him favourite? Not massive, but at any point he could switch Bivol’s lights out. But I am going Bivol in a unanimous decision but a very close fight. What I mean by that is the rounds will be enough. I am going with the matador in this fight but I wouldn’t put my house on it.”
Bivol’s promoter Eddie Hearn: “When you fight Beterbiev, it’s like a ticking time bomb. The speed of that time bomb depends on the opponent. Dmitry has the ability to make that tick really slow. How slow can he make it tick? The boom point is 12 rounds.
“Beterbiev is going to get you, but how quick can he get you and how long do you have left when he does get you? The only person who can beat Beterbiev right now is Bivol.”
Former world champion Hannah Rankin: “This is a brilliant match-up of two fighters who have been waiting to fight for a long time and now all the marbles are on the line.
“I’m going with Bivol on points as he is the younger and fresher of the two with less wear and tear on his body. However, I wouldn’t be surprised if Beterbiev does drop Bivol during the fight.”
Saudi welterweight Ziyad ‘Zizo’ Almaayouf: “Call me crazy, but I say Bivol takes this very swiftly actually. There’ll always be danger from Beterbiev, but I see Bivol controlling the distance too well.”
Former world champion Regis Prograis: “I’ve got to bet on Bivol, he’s a beast. That’s a really good fight, the only bad thing is that they don’t have big names for those outside of boxing fans. But that’s a badass fight.”
Retired world title challenger Paul Smith: “From the start I’ve said Beterbiev and I think it could be a late stoppage. I don’t think there’s anyone out there who can keep him off them. He’s too relentless.
“Bivol is obviously exceptionally good and has unbelievable footwork so I wouldn’t be surprised if he did win. But I’ve always felt Bivol, apart from against Saul ‘Canelo’ Alvarez, has done just enough to win fights and I’m not sure that will be enough against Beterbiev. I don’t think he hits hard enough to command Beterbiev’s respect.”
English cruiserweight champion Viddal Riley: “I’m going with Bivol. He has the better movement, better flow and will be able to outbox Beterbiev for the majority of the rounds. But as we get into the later rounds, it will turn into a war.”