Conor McGregor says Floyd Mayweather is “scared” in the build-up to their multi-million dollar fight on 26 August in Las Vegas.
Mayweather, 40, said on Thursday that the bout “won’t go the distance”.
Speaking at a media workout on Friday, McGregor, 29, said: “You know what I heard him say yesterday? Nothing. I saw a man scared… I saw a man trying to talk himself out of it.
“I’m preparing to win, and to win in devastating fashion.”
UFC lightweight champion McGregor, who has never boxed professionally, is due to face Mayweather in a 12-round super welterweight boxing bout that could earn both men about $100m.
Mayweather, who retired in 2015 with an unblemished 49-0 record, has beaten boxers such as Ricky Hatton, Manny Pacquiao and Oscar De La Hoya during his professional career and is aiming to surpass Rocky Marciano’s mark and move to 50-0.
But McGregor said he expects to defeat the American within 12 rounds.
The Irishman said: “The story of the fight will be waiting to see who takes a step back first.
“We all know Floyd is a great defensive boxer and he isn’t going to keep coming forward like he says. If he does, it’s going to be a quick night for me.
“I have been boxing my whole life so this is not a new experience for me. The critics say that Floyd has been there before. They discredit my training, my skills, my accomplishments.
“This isn’t new to me though, just like boxing isn’t new to me. I will rule over MMA and boxing with an iron fist after August 26.”