A little — or even a lot — of rain wasn’t going to dampen “Smoke on the Water” or “Hell Bent for Leather” on Thursday night at Treasure Island Amphitheater. A tornado warning, however, was another matter.

The Red Wing area gambling hub’s doubleheader of the classic British hard-rock groups Deep Purple and Judas Priest still went on as planned, but the concert had to be moved indoors with a few hours’ notice — from the casino’s lively new outdoor venue to its stale old conference-room-like event center.

It was the rare case of a venue being happy about poor ticket sales; only about 3,000 tickets had been sold in advance, just a hair over the capacity inside.

The move proved to be a wise one, too: Fans’ phones repeatedly chimed in unison with tornado-warning alerts as they waited in a long line for the event center to open.

Once inside, seating was a free-for-all, and space was cramped. But those hiccups were more than compensated for by the novelty of seeing the two longtime arena-rock gods squeezed into a casino ballroom made for oldies acts — older than these oldies, that is.

Aside from Priest’s Armory gig in April, both groups had been scarce in Minnesota of late. And let’s face it, chances are slim they’ll still be rocking deep into the next decade.

Deep Purple dubbed its current outing the Long Goodbye Tour, and is dedicating songs to keyboardist Jon Lord, who died in 2012. Priest, meanwhile, had to hire a new guitarist at the start of this tour after the last of its two original slingers, Glenn Tipton, was diagnosed with Parkinson’s disease.

“The rock ’n’ roll show must go on, so here we are,” Priest singer Rob Halford unflappably declared after his band’s opening songs, “Firepower” and “Delivering the Goods” — the former the title track of their new album, the latter a 40-year-old deep cut that truly delivered.

With the band’s older “new” guitarist Richie Faulkner dutifully handling the scorching leads, "Firepower" sparked a rave response. So did the fist-pumper “No Surrender,” after Halford injected even more spirit into it by dedicating it to Tipton, saying, “It’s about facing challenges in life.”

A man who’s fought through his own challenges — almost single-handedly batting down homophobia in metal, for one — Halford was the most godly rock god of the night. Throughout the 80-minute set, he wailed and wallowed with a range that singers a third his 67 years would envy. Of course, he had plenty of help from the crowd singing the old faves later in the set, including “You’ve Got Another Thing Comin’,” “Breaking the Law” and a truly hellish “Hell Bent,” for which he rode a motorcycle onto the ballroom stage. Take that, Engelbert Humperdinck!

Deep Purple’s Ian Gillan — the band’s best-known vocalist of the ’70s — didn’t muster the resiliency that Halford had at the microphone. He squawked the high notes in the hard-charging opening song “Highway Star,” and he avoided the ubiquitous “My Woman From Tokyo” altogether — perhaps left out because it’s too hard to sing, or maybe for 2018 political correctness.

Gillan’s band still brought plenty of fire, though, with original drummer and bassist Ian Paice and Roger Glover anchoring a deep groove in “Space Truckin’ ” and “Strange Kind of Woman.”

Guitar hero Steve Morse — who replaced Ritchie Blackmore in the band two decades ago — offered the same redwood-thick tones and stormy leads as his predecessor, with plenty of his own compelling flourishes, especially in the bluesy fave “Lazy” and ’80s hit “Perfect Strangers.”

By the time the Purple players got to “Smoke on the Water” and the hazy early hit “Hush” at show’s end, the crowd seemed pleasantly blown away — which sure beat being literally blown away had the concert stayed outdoors.