Kamaru Usman claims he offered Conor McGregor a shot at the welterweight title but was met with silence.
Former featherweight and lightweight champion McGregor returned to the Octagon in January after a 15-month absence and knocked out Donald Cerrone at welterweight in just 40 seconds.
It was the first of three fights the Irishman was planning in 2020, but the coronavirus pandemic scuppered that strategy and he announced he had decided to retire in June.
Usman says he was willing to face McGregor but received no response from the 32-year-old.
Asked about a possible fight with McGregor, Usman told TMZ Sports: "That's just a lost cause. The only time Conor gets up to the welterweight division is if he's able to pick and choose lightweights with double-digit losses that he can pick off and not have to cut weight to get down to lightweight.
"If Conor wants a piece at welterweight, he knows who the king of the division is. I'm not going anywhere. I'm right here.
"I've offered him a shot. I said, 'Hey, do you want it? It's yours, you got it'. And he was silent.
"Days later, Anderson Silva says, 'Hey, let's do it'. And he responded and said, 'Yeah'. It's clear right there to know if Conor is willing to step up and take a fight like this, but I think he's a wise guy so he should just stay retired.
"If Conor wants to put my name in his mouth, then come get some. Anybody wants to say something about me then just come get some. That's it. I'm the king. I'm not in a position where I wanna call people out, I'm not doing that."
However, Usman would be interested in taking on former welterweight and middleweight champion Georges St-Pierre, who has not fought since November 2017 and has been retired for the past 18 months.
He said: "As far as Georges St-Pierre, yeah I would love to test myself against a guy like that. But, as far as Conor [goes], if he wants a piece, he can come talk to the king.
"Those aspirations of becoming a welterweight champion severely turn into nightmares when you step inside that Octagon with me."