Oleksandr Usyk claimed "the greatest prize in the world" as undisputed champion, but predecessor Lennox Lewis admits the harder battle is holding onto the title.
The Ukrainian made history as he united all four belts for a second time against Daniel Dubois in July, but looks set to be ended once again by factors beyond the ring.
Having been ordered to face Joseph Parker by the WBC, Usyk has sought an extension, but could yet be forced to surrender his belt if he does not fight the New Zealander.
Now Lewis, who was the last man to unite the glamour division, has offered his thoughts on the difficulty of not only achieving the feat, but then retaining the crown too.
"There's always somebody out there that says they can beat you and you have to show them that they can't beat you," he told Sky Sports News.
"It's hard getting to be the undisputed heavyweight champion of the world. But it's even harder to keep it.
"It is the greatest prize in the world. It meant to me that I was the best. There were so many different champions, so many different belts
"But being undisputed marks me as the number one guy and that's what I wanted to do. I wanted to bring all the belts together.
"When they said, 'that guy's heavyweight champion and that guy's heavyweight champion' - no. There can only be one guy.
"That was my aim to go out and get that championship and be undisputed heavyweight champion of the world."
Lewis saw his own reign as undisputed champion curtailed by his decision to face WBC mandatory challenger Michael Grant rather than WBA contender John Ruiz.
The latter took the case to court, who ruled in his favour, and saw the Briton fail to mount a single defence of all three titles.
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