Chicago performs at the Austin360 Amphitheater on Saturday, June 17, 2017. Scott Moore for American-Statesman

Take a hit-machine rock band celebrating 50 years since its inception, add another band that’s been around for nearly as long, and you have the makings of a marathon concert. Welcome to Saturday night’s Chicago and Doobie Brothers double bill at the Austin360 Amphitheater, which ran for four hours as both bands recapped repertoires from their 1970s heydays and beyond.

PHOTOS: Chicago and the Doobie Brothers at Austin360 Amphitheater

With more than 20 top-40 hits in the ’70s and another dozen in the ’80s, Chicago couldn’t play less than two hours and still fit in all the songs longtime fans wanted to hear. Thus this rare occasion of a twin bill in which the headliner also took an intermission, resulting in two breaks during what became an endurance test. The vast majority of attendees made it all the way to the end, rewarded by Chicago’s encore rendition of its 1970 hit “25 or 6 to 4.”

There’s no other band quite like Chicago in all of rock ‘n’ roll, even if the current touring lineup features just three of the seven founding members. In concert, they’re a kinetic whirlwind of activity: The three horn players, two keyboardists, bassist and guitarist constantly shift spots onstage, never anchored to the same spot like musicians typically are in a rock show. The horns are the key, frequently placed front-and-center and driving the music’s direction.

The band faces a lot of challenges in rendering well-known material recorded by vocalists no longer in the lineup. The presence of original keyboardist Robert Lamm, the closest thing Chicago has to a frontman, is essential; favorites such as “Does Anybody Really Know What Time It Is” and “Saturday in the Park” fared well because he sang on the hit versions.

One more bit from last night with #ChicagoTheBand at @Austin360Amp: "Saturday in the Park." Yes it was! #ClassicRock

— Peter Blackstock (@Blackstock360) June 18, 2017

Bassist Jeff Coffey, the group’s newest recruit, did a credible job covering for Peter Cetera, who left for a solo career in the 1980s. Cetera’s voice is so central to numbers such as “Just You ‘n’ Me,” “Searchin’ So Long” and “If You Leave Me Now” that it’s hard for even a close imitation not to sound like a Chicago tribute band. It helps that horn players James Pankow — who wrote two of the aforementioned three numbers — and Lee Loughnane are founding members. (Saxophonist Ray Herrmann filled in admirably for Walter Parazaider, who still records with the band but doesn’t tour.)

Pankow is easily the most fun to watch. He’ll be 70 in August but he still prowls the stage with the spirit and zeal of a teenager, often exhorting the crowd with fist-pumps and wide smiles as he pours all of his breath into propulsive trombone blasts. Flanking Lamm on the riser at stage right, keyboardist Lou Pardini covers some of the vocal parts that were silenced when guitarist Terry Kath died from an accidental gunshot in 1978. In Kath’s place on guitar is Keith Howland, who’s been with the band since 1995. Drummer Tris Imboden and percussionist Walfredo Reyes Jr. anchor the unit with rhythmic precision and creativity.

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If the band felt compelled to play its biggest hits, sometimes that was to its detriment. Prefacing two of its most regrettable 1980s power ballads, “Hard Habit to Break” and “You’re the Inspiration,” Pankow explained almost apologetically that “something happened in the ’80s; music changed” and a producer (David Foster) steered them in a different direction. Commercially it was a success, as the hits kept coming. But in retrospect, those tunes pale in comparison to the group’s groundbreaking early work.

As such, perhaps the most illuminating part of the night was an early section featuring Pankow’s seven-part extended suite “Ballet From a Girl in Buchannon,” from their 1970 sophomore album. It encompassed two of Chicago’s best-known songs, “Colour My World” and “Make Me Smile,” but also featured instrumental sections and other vocal numbers that underscored just how ambitious the band was at the outset. A set-opening volley of songs from the 1969 debut “Chicago Transit Authority” also stood out, stressing the historical significance of this 50th-anniversary tour.

Founding Doobie brothers Patrick Simmons, left, and Tom Johnston, right, with saxophonist Marc Russo at Austin350 Amphitheater on Saturday, June 17, 2017. Scott Moore for American-Statesman

The 75-minute opening set from the Doobie Brothers likely struck the same nostalgic chord with this largely baby-boomer audience, even if the two groups are vastly different in style. Where Chicago is uptown horn grandeur drawing on jazz and theater, the Doobies are backwoods roots charm grounded in folk and country. But in the 1970s, all that stuff mixed together somehow made perfect sense on the radio, and so it still works well in a rock ‘n’ roll package tour.

As with Chicago, the Doobie Brothers’ present incarnation contains a significant minority of original members. Only guitarist-vocalists Patrick Simmons and Tom Johnston remain from the founding lot, and the absence of distinctive singer Michael McDonald leaves the same kind of hole that Chicago has without Cetera. But the Doobies benefit from a couple of major-league ringers in their lineup: Bassist John Cowan is an extraordinary singer with a New Grass Revival pedigree, and keyboardist Billy Payne is legendary for his work with Little Feat. Multi-instrumentalist John McFee, who joined in 1979, rounds out the lineup along with more recent recruits Marc Russo on saxophone and Ed Toth on drums.

The Doobies got the crowd hoppin’ early as the sun gradually sank below the lip of the amphitheater’s back lawn. Favorites such as “Long Train Running” and “Rock Me Down the Highway” sounded fresh and full of energy, while the more acoustic “Black Water” brought a wave of appreciative applause. The built-in shout-out to “the Lone Star State” made “China Grove” an obvious crowd-pleaser here, while Simmons and Cowan did a fine job in covering McDonald’s vocal stamp on “Takin’ It to the Streets.” Capping a brief encore, the band asked everyone to sing along on the easygoing finale “Listen to the Music,” then took a well-deserved bow before Chicago’s turn in the spotlight.

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The Doobie Brothers perform at Austin360 Amphitheater on Saturday, June 17, 2017. Scott Moore for American-Statesman