When Ray Benson throws a birthday party, you probably want to be there if you can. Two years ago, Gary Clark Jr. showed up to jam. Last year it was George Strait. So the thousand or so folks who filled up advertising firm GSD&M’s back yard on Tuesday probably knew something special was coming.

PHOTOS: Willie Nelson and guests play the Ray Benson Birthday Bash

They got it in the last hour. First the Avett Brothers joined Benson and his Asleep at the Wheel bandmates, playing a mini-set focused on songs from Bob Wills’ catalog. The topper was Willie Nelson, who ambled onstage at about 9:30 to swing this party home in true Texas style.

Willie Nelson, right, performs with Ray Benson as the Avett Brothers’ Seth Avett joins in behind them at GSD&M on Tuesday, March 14, 2017. JAY JANNER / AMERICAN-STATESMAN

From his trademark opener “Whiskey River” to the equally classic closer “On the Road Again,” Nelson whirled through about a dozen tunes that put smiles on everyone’s faces. He tossed in a few curveballs, too, including “I Woke Up Still Not Dead Again Today” from his upcoming album due this spring, and a great jazz instrumental that found brothers Scott and Seth Avett watching Willie’s fingers on his trusty guitar Trigger with wide-eyed wonder and awe.

WATCH: More video of Willie Nelson at Ray Benson birthday bash

Maybe best of all was a heartfelt cover of Townes Van Zandt’s “Pancho & Lefty,” with Benson taking the final verse. It wasn’t the first Townes tune sung at the show; earlier, Charlie Sexton and Shannon McNally delivered a beautiful take on “No Place to Fall.” hot on the heels of Benson and Wade Bowen’s version of “Dublin Blues” by Van Zandt’s close friend Guy Clark.

The night was full of great song selections. The Avetts’ set featured Merle Haggard’s “Mama Tried,” a nice follow-through on Bowen and Randy Rogers’ earlier rendition of the Hag’s “Workin’ Man Blues.” Carolyn Wonderland, joined by Shelley King (and later Marcia Ball as well), lit the joint on fire with Blind Willie Johnson’s “Nobody’s Fault But Mine,” while Brennen Leigh got things rolling early with Lefty Frizzell’s country classic “I Love You a Thousand Ways.”

A mid-show highlight was Sunny Sweeney’s sweet rendition of Chris Wall’s 1980s ballad “I Feel Like Hank Williams Tonight,” recently revived for her new album that came out last week. A half-hour break between the early guests and the surprise headliners allowed patrons to visit booths where donations were being accepted for the Health Alliance for Austin Musicians, Benson’s designated beneficiary for the event.

READ MORE: Austin’s musicians show their support for HAAM