Tyson Fury issued a warning to Cristiano Ronaldo, insisting "no matter who you are, age will get you eventually".
The Portugal captain, who entered the World Cup fresh from his acrimonious departure from Manchester United, has scored just once in four appearances in Qatar.
That goal came from the penalty spot against Ghana in his nation's opening match.
Having been criticised by head coach Fernando Santos for his reaction to being substituted during the Group H clash with South Korea, Ronaldo was then dropped for Portugal's comprehensive 6-1 victory over Switzerland in the last 16.
After being left out of the starting line-up, A Selecao have subsequently strongly denied reports the 37-year-old threatened to quit the squad during what is likely to be his swansong at the finals.
Fury - the WBC heavyweight champion and a lifelong United fan - insisted he empathises with Ronaldo, who he feels is no longer at the peak of his powers.
The undefeated boxer drew comparisons with his iconic victory over Wladimir Klitschko in 2015.
"At the time, beating Klitschko was the Everest," Fury told talkSPORT.
"I was the heavy underdog, people thought I was going to get smashed to pieces, and he was the unified champion for 11 years and at some point, he thought he was unbeatable.
"But I went over to Germany and beat him. In this game, it's not about how much better somebody is than the other person.
"A lot of it has to do with natural stuff, like age, and at 38 years old, you are on the decline - whether you want to admit it or not.
"If you look at Cristiano Ronaldo, he's 37 years old, he's had the best doctors, the best physiotherapists - he's lived like an egg in Mr Kipling's cake.
"But even Ronaldo is on the slide now.
"So even someone who has looked after himself like that, it catches up to you - so, imagine a boxer who has been punched to pieces for the last 20 years!
"The age factor will not wait for anybody, no matter how you've lived.
"Whether you've never had a drink in your life, if you've never run on any hard surfaces, no matter who you are, age will get you eventually."
Ronaldo will hope to be reinstated to the starting line-up when Portugal face Morocco in the quarter-finals on Saturday, with his team aiming to reach the semi-finals for only the third time.