Khamzat Chimaev Believes Usman, Not Robert Whittaker, Toughest Test Of Career

Khamzat Chimaev has his hands full with Robert Whittaker -- the former UFC middleweight champion -- but, he doesn't believe it'll be the toughest test of his career ... 'cause that honor belongs to Kamaru Usman.

TMZ Sports talked to 30-year-old Chimaev from Abu Dhabi just a few days before his UFC 308 co-main event against Whittaker ... when we asked him if he believed Robert was the most dangerous fighter he's faced.

"I think Kamaru is a better fighter than this guy, but we will see in the cage. I can't answer for that right now, so we'll see," the 13-0 fighter told us.

Of course, Usman and Chimaev fought at UFC 294 ... and it was epic. Khamzat won the three-round scrap by decision, but it was a war ... and many people believed if the fight had been 5 rounds (like a championship bout), Usman would've won the fight -- which he took on short notice (under 2 weeks).

But, Whittaker's no slouch. He's 26-7 ... and has beaten some of the most dangerous men in the world, including Paulo Costa, Yoel Romero (twice), Kelvin Gastelum, Derek Brunson, and Darren Till.

Despite being ranked 13th, Khamzat, who has dealt with long layoffs over health issues, believes he can, with a win over #3 ranked Whittaker, show UFC matchmakers he's "the best guy in the world."

As for who he'd then like to face ... he wants the champion, Dricus Du Plessis.

"I want to fight Dricus because that guy a lot of good fighters he's beat. So I want to meet him. And everyone says he's got a strong wrestling background. I want to see who's best," Chimaev said.

But, first things first.