At Xfinity Center, Phish shows band is always evolving

Phish’s lead guitarist Trey Anastasio at the Xfinity Center Friday.Robert E. Klein for the Boston Globe/Globe Freelance

MANSFIELD — For most bands on the summer shed circuit, a good night means clean execution of a well-practiced show. The highs and lows are carefully structured, the business all cresting somewhere near the end with delivery of the expected hits and favorites. That’s fine; it’s how you do big-deal rock and roll. Hello, Cleveland.

It’s to Phish’s great credit that the Vermont-spawned jamband builds uncertainty into each performance, even when playing to a sold-out, Friday night audience at the Xfinity Center. Each show is both a self-contained universe — with unique set list and uncertain improvisational yield — and part of an ever-evolving conversation with the fan base and the band’s own catalog.

Not all of the uncertainty this night came in the form of bold artistic choices: A catastrophic failure of the PA sound system rudely interrupted what was shaping up to be the best set of the tour, and the band never recovered momentum.

But Phish offered a checklist of the things it does best, from booty-wriggling opener “Party Time” to a positively chill-inducing, a cappella take on David Bowie’s “Space Oddity.” Though the playing on this tour has had sloppy stretches, the band sounded pretty sharp all night.

Trey Anastasio, whose technical facility and improvisational imagination made him one of the greatest rock guitarists of the 1990s (and perhaps beyond), has shown only intermittent interest this tour in blowing the doors down. And so in a methodically unhinged cover of Talking Heads’ “Cities,” Anastasio ditched his ax to fiddle tentatively with percussion textures on a MIDI controller known as the Marimba Lumina, while bassist Mike Gordon subbed on guitar.

The sequence was entertainingly diverting, but Phish played more to its strengths in the second set with a hard-grooving “Light.” Gordon and drummer Jon Fishman swung stylishly as Anastasio peeled off busy but rhythmically solid lines and Page McConnell offered spicy accompaniment on Hammond B-3 organ.

Shortly thereafter, the sound in the house cut out. Phish left the stage for five very long minutes before things were sorted out in the pavilion, but reportedly the sound was never restored to the lawn.

The spell was broken, the band thrown off stride; a few perfunctory run-throughs filled out the set, before a euphoric “Slave to the Traffic Light” capped things.

Phish is 33 years into a Rock and Roll Hall of Fame-caliber career and its story is still unfolding, night by night. Of that you can be certain.

Watch Phish’s a cappella cover of David Bowie’s “Space Oddity”:

Phish

At: Xfinity Center, Mansfield

Friday