Defending all of the WBA (Super), IBF, WBO, and The Ring belts for the first time since his own stunning victory over the great Vasiliy Lomachenko last year, Lopez was knocked down in the first round in the main event at New York’s Hulu Theater on Saturday night before losing an enthralling split decision.
In a bout that was delayed on multiple occasions and with no love lost between the two fighters, IBF mandatory Kambosos Jr - who himself responded to being knocked down in round 10 - was favoured on two of the three judges’ scorecards, 115-111 and 115-112, while the other scored in favour of Lopez, 114-113.
“I believed in myself, I backed myself,” Kambosos Jr said after his upset. “I said it time after time: You might not believe it, but I believe in myself. And look at me now. I’ve got all the jewels.
“I’m not the king, I’m the emperor because I come to every other country and I take them out one by one.”
Lopez - who was bloodied with a nasty gash above his left eye after a punishing contest - took the microphone after the bout and insisted to a disapproving crowd that he had clearly won.
“He’s a hell of a fighter, but I won tonight, everybody knows that,” Lopez said.
“The referee raised my hand. I won tonight, I don't care what anybody says, I won tonight. At the end of the day, I’ve been here, I done that, I want to thank God, I want to thank everybody that came out tonight, look, I ain’t no sore loser, I take my wins like I take my losses.
“At the end of the day man I’m a true champion, I did what I had to do, and I went out there and did my best. I don’t care what anybody says man, I am as real as they come, and this is the takeover man, we don’t stop, we keep coming.”
With Saturday’s bout a mandatory for the IBF, there is crucially no rematch clause.
Kambosos Jr will no doubt now have his eyes fixed on further huge fights against the likes of Devin Haney, Gervonta Davis and Joseph Diaz.
Meanwhile, it remains to be seen if Lopez still plans to move up to super-lightweight and challenge Scotland’s undisputed world champion Josh Taylor.