Guns N' Roses guitarist Slash still has nothing but admiration for Axl Rose's performances filling in on lead vocals for AC/DC in 2016 on the band's 'Rock or Bust' tour.

After longtime frontman Brian Johnson had to step away from live performing to address a hearing issue, AC/DC tapped Rose to help complete the band's scheduled tour dates.

But not only did Rose fill-in for Johnson; he did so while in the midst of GNR's reunion tour. The timing might have seemed awkward from the outside, but the GNR camp understood how much the opportunity meant to Rose and they supported him giving it a shot.

"I came out to a gig in London, and it was phenomenal," Slash recalled in a new interview with Classic Rock magazine. "I was blown away, especially when he sang the Bon Scott stuff. That was a very proud moment, actually."

Slash was asked how he felt when Rose first shared news of the opportunity.

"In the moment, when it first came up, it was a little jarring, I have to say," he conceded. "I was super proud that he was doing it, but how that was gonna happen and go right into the Guns thing, I really didn't know. But anyway, it worked out. So it wasn't like a big deal."

Rose assured his GNR bandmates that he could make it work with both tours. Slash couldn't blame him; the AC/DC opportunity was a dream gig.

"The fact that he got asked to do that was very cool," he added. "And he worked his ass off doing it, too. He really adhered to the whole AC/DC regimen and pulled it off."

There was a time when Rose would not have been able to handle both jobs, but Slash notes that the singer had grown a lot since the '90s — the last time Slash was in GNR.

"In all these years that we've been apart, he's become super-f---ing professional," Slash said. "And he's never missed a beat during this whole time. So it's been great. There has been a sort of synergy that's been happening this last six years that we never had in our first incarnation."

Duff McKagan has also praised Rose's work ethic in recent years, telling Q104.3 in 2019 that the singer's preparedness has brought the best out of everyone around GNR.

"...[H]e's an animal," McKagan explained. "He's in the gym. He starts and hour-and-a-half before the show vocal warmups. We do a three-and-a-half-hour long show. He broke his foot at the beginning of the tour; he didn't say, 'Let's stop.' He said, 'Let's continue, I'll do it sitting down.' ... singing sitting down is super hard."