Retired boxing legend Floyd Mayweather, who held 11 world titles in five divisions, made a second victorious one-match comeback on Monday, beating a Japanese kickboxing star in the first round with a technical knockout (TKO.)

Former UFC lightweight champion and whiskey entrepreneur Conor McGregor has jeered another big name in the game, Floyd "Money" Mayweather, sending fans into a meltdown.

"That 9 milli won't keep you on top of my list for long, kid," McGregor wrote in a tweet.

Although the Irishman did not name the "kid", he seemingly referred to Mayweather, the renowned US boxer who made $9 million from a first-round victory over Japanese kickboxer Tenshin Nasukawa in an exhibition boxing match on 31 December.

Mayweather, 41, the flawless five-division boxing world champion, retired from sports in 2015. Last time he dusted off his gloves was in 2016, when he defeated McGregor via a tenth-round technical knockout, extending his professional record to 50-0.