Oleksandr Usyk is running out of adversaries, especially foes that equate to massive events.

The undisputed heavyweight champion of the world has beaten Tyson Fury, Anthony Joshua, and Daniel Dubois twice each over the last four years.

The most legitimate threat left for Usyk (24-0, 15 KOs) is WBO mandatory challenger and interim titleholder Joseph Parker (36-3, 24 KOs).

The sanctioning body ordered the fight last month, but last week it was revealed that Usyk requested an extension period due to an injury.

Parker is riding a six-fight winning streak, with the last three victories coming against Martin Bakole, Zhilei Zhang and former WBC champion Deontay Wilder.

There is the surging Agit Kabayel (26-0, 17 KOs), The Ring's No. 3-ranked contender, who stopped Zhang, Frank Sanchez and Arslanbek Makhmudov over his last three fights, as well as the fast-rising 20-year-old phenom Moses Itauma (13-0, 11 KOs), who just demolished longtime contender Dillian Whyte in one round earlier this month.

Hall of Fame fighter and former four-division champion Roy Jones Jr. – who once held a heavyweight title as well – believes Usyk will retire undefeated.

"I don't see anyone beating him at this point," Jones told The Ring. "But the most likely suspect is Joseph Parker. It's the only fight that is a little bit of a challenge because Parker has boxing skills, hand speed, he knows how to maneuver like a small man, and has trainer Andy Lee, who knows what he is doing as well."

It remains to be seen how Usyk will maneuver his career moving forward. The 38-year-old Ukrainian southpaw has been hinting at retirement during the buildup of his fights over the last 18 months.