The lineup for the Outlaw Music Festival tour at the Mann Center on Saturday was so formidable that the headliner was almost an afterthought.

The make-sure-you-get-there-early opener was Nashville songwriter Margo Price, finally out on the road again to spread the word about last year’s exemplary That’s How Rumors Get Started.

Price was followed first by guitarist Warren Haynes’ Southern rock band Gov’t Mule, and then by full-of-surprises country tough guy Sturgill Simpson, backed by a dazzling band of bluegrass musicians.

And, oh yeah, the last act to hit the stage: some guy named Willie Nelson.

Fans could be forgiven for tempering expectations for Nelson’s closing set. Sure, the “Red Headed Stranger” is the essential embodiment of the Outlaw brand, dating to when he and Waylon Jennings flipped the bird to Nashville and planted a nonconformist flag in Texas in the 1970s.

But on Saturday night at the nearly sold-out Mann — where proof of vaccination or a negative COVID test was required for entry, and mask compliance was better than at last week’s Made In America festival — Nelson was allotted only an hour of stage time, 15 minutes less than both Simpson and Gov’t Mule. Touring with his Family band, he was seated with pianist sister, Bobbi, on his right and guitarist son, Micah, on his left.

Micah eased the load for his father by taking lead vocals on four songs, including the country gospel standard “Keep on the Sunnyside” and the brand new “If I Die When I’m High I’ll Be Halfway to Heaven,” which Micah said he wrote after his father suggested the title.

Opening as always with “Whiskey River,” as an American flag backdrop was revealed — replacing an Outlaw tableau featuring a tour bus trailed by clouds of smoke disappearing into a tree — Willie took a little while to warm up.

He set the mood with the Zen koan “Still Is Still Moving to Me,” then eased into wistful, melancholy standards, some of which he wrote himself, such as “Crazy” and “Night Life,” and others, such as “Always on My Mind,” that he has made so indelible you just assume he wrote them.

Like an aging athlete finding his footing, Nelson got stronger as the show went on, his vocals more robust, the gypsy-jazz leads he squeezed out of his acoustic guitar Trigger friskier. The pairing of Hoagy Carmichael’s (by way of Ray Charles) “Georgia on My Mind” and Billy Jo Shaver’s “Georgia on a Fast Train” was particularly winning.

And by the time the encores rolled around, with all the evening’s featured performers joining him for the country gospel sing-alongs on “Will the Circle Be Unbroken?” and “I’ll Fly Away,” Nelson was commanding the stage from his seated position.

So much so that he decided to throw Mac Davis’ mock boastful “It’s Hard to Be Humble” as a closer on what was an evening of 5½ hours of performances. “To know me is to love me,” Nelson sang playfully. Indeed it is, and all the more so now, considering how vital and indomitable of an artist he remains at age 88.

Simpson preceded him with 75 minutes of bluegrass virtuosity drawn from his two 2020 Cuttin’ Grass albums and the new Ballad of Dood & Juanita, a narrative concept album that he wrote and recorded in a week.

The standouts among the peerless group of pickers were mandolin player Sierra Hull and fiddler Stuart Duncan. The all-star players cohered as a unit, and were super-tight — in contrast to Nelson’s band’s lovable looseness — despite it being only the second show of the tour.

But as much as the focus is on the ensemble, it’s Simpson who makes it work, not only with his uncompromising artistic vision, but also because his chesty baritone is as forceful as the band revving behind him.

Gov’t Mule’s set was the sluggish stretch of the evening. With the exception of the superb closer “Soulshine,” which Haynes wrote during his Allman Brothers days, the Mule’s jam-band blues-rock tended to drag. Action picked up during Neil Young’s “Keep on Rockin’ in the Free World,” on which Haynes was joined by Micah Nelson, and a cover of Etta James’ “I’d Rather Go Blind.”

Price was fabulous in a too-short, 45-minute set, leading a seven-piece country-rock band featuring her husband, Jeremy Ivey, on guitar. Standout moments included the feminist anthem “Four Years of Chances” and a closing honky-tonk troika of her own, “Hurtin’ (On the Bottle),” Merle Haggard’s “I Think I’ll Just Stay Here And Drink” and Nelson’s “Whiskey River.”

The highlight of the set (and maybe the whole show) came earlier, however, when Price used a song written by Philadelphians John Madara and David White in 1963 to target lawmakers in the Lone Star State who passed restrictive antiabortion legislation that took effect this month. “This just seemed appropriate with everything that’s going on in Texas,” she said, before making the message of Lesley Gore’s “You Don’t Own Me” absolutely clear.