"I like my chances," Koepka said, adding that "(Johnson's) only won one (major)."
Upon hearing of Koepka's comments, Rory McIlroy defended Johnson's Hall of Fame-caliber career.
"I certainly try to respect everyone out here," McIlroy said. "Everyone is a great player. If you've won a major championship, you're a hell of a player. Doesn't mean you've only won one; you've won one, and you've had to do a lot of good things to do that."
McIlroy also noted Koepka's fewer PGA Tour wins compared to Johnson's total.
"(It's) sort of hard to knock a guy that's got 21 wins on the PGA Tour, which is three times what Brooks has," he said.
McIlroy also said Koepka may have been playing "mind games," but he doesn't think that would impact Johnson in any way.
"(Koepka is) trying to do it to the wrong person," the Tour veteran said. "I don't think DJ really gives much of a concern (to) that."