Tony Bennett and Lady Gaga Dueted. Their Stars Were Reborn.

It’s still one of the most impressive flexes I’ve ever seen a musician pull off live — and at the age of 88, no less.

In 2015, during the third of four sold-out shows at Radio City Music Hall, Tony Bennett and Lady Gaga were sharing a bill, promoting their chart-topping, cross-generational 2014 duets album, “Cheek to Cheek.” They had a light, snappy chemistry on the songs they sang together, but the best parts of the night were their solo sets, each inviting their respective fan bases — Bennett’s tasteful traditionalists and Gaga’s sartorially zany but spiritually sincere Little Monsters — into the other’s world.

For most of the concert, they’d been playing with a full band and orchestra, but for one number during his own set, Bennett summoned a single guitarist to join him in the snug radius of a spotlight. He told us the song was dedicated to his “best friend, Frank Sinatra,” and launched into a velvety rendition of “Fly Me to the Moon,” holding the microphone down at his side rather than bringing it to his lips. A few lines in, he set the mic down atop a piano and sang the rest without any amplification at all. The entire venue was suspended in a hush stillness, and Bennett’s voice was so strong and clear that you could hear every crystalline note, every enunciated lyric, even in the cheap seats.

It was spellbinding, and so quintessentially Tony Bennett: the unshowy elegance, the inevitable name-dropping and, above all, the ease with which he suddenly transformed from the rat-a-tat every-crooner into a phenomenally gifted belter who could project like an opera singer.

In August 2021, while battling Alzheimer’s disease, Bennett, who died on Friday at 96, made his final public appearance on that very same stage, again with Lady Gaga. He once again demonstrated strength and resilience, this time by simply performing at all. A poignant “60 Minutes” segment captured Bennett’s struggles in rehearsals but his ultimate triumph when he took the stage. In run-throughs, Gaga said, “He called me ‘sweetheart.’ But I wasn’t sure he knew who I was.” She witnessed a startling transformation, though, anytime the band struck the opening notes of another song and Bennett began to sing.

“When the music comes on, something happens to him,” she said. “He knows exactly what he’s doing.”

Here was the final act of an unlikely collaboration that had changed the trajectory of each musician’s career. When Gaga first linked up with Bennett for “Cheek to Cheek,” some skeptics saw it as nothing more than a savvy distraction, a way for a wild pop instigator to rebrand as a throwback jazz singer in the wake of her first major flop, the overblown (if, in hindsight, somewhat underrated) 2013 album “Artpop.” But the gusto, reverence and musical intelligence she brought to her work with Bennett undoubtedly won her fans and respect from an older generation of listeners. As I was filing out of Radio City that night in 2015, I couldn’t keep track of how many people I’d heard muttering versions of, “I had no idea that Lady Gaga could actually sing!”

Bennett was no stranger to cannily timed reinvention, either. He stormed MTV when he was in his late 60s, recording an “Unplugged” album that featured collaborations with Elvis Costello and K.D. Lang, and that eventually won him a Grammy for album of the year. He sang with more eclectic and, in some cases, even younger musicians on his series of “Duets” albums, from 2006 to 2012. He found a kindred spirit in Amy Winehouse, but their connection was short-lived. Their great rendition of “Body and Soul,” for “Duets II,” was the last thing she ever recorded. It was released as a single posthumously, on what would have been Winehouse’s 28th birthday.

Gaga satisfied Bennett’s desire to stay active and involved with a younger generation of musicians, and her professional stability made her into the most committed of his duet partners. But Gaga has also said that Bennett’s mentorship “saved” her life. The then-octogenarian’s example allowed her to think beyond the successes or failures of the present moment, and to value a musical career’s longevity. “I was so sad. I couldn’t sleep. I felt dead,” Gaga said of the time before “Cheek to Cheek.” “And then I spent a lot of time with Tony. He wanted nothing but my friendship and my voice.”

It’s not that their voices or energies always blended particularly well — Gaga brought an antsy theatricality to their collaborations, while Bennett’s voice seemed to get even more laid-back and free of artifice as he aged — but the mutual admiration they shared was genuine enough to open the minds of their respective audiences and generational cohorts. With their Grammy-winning 2021 album of Cole Porter covers, “Love for Sale,” Bennett seemed to be passing the baton to Gaga, deeming her capable of continuing his lifelong task of keeping the Great American Songbook alive. And Gaga, in turn, was telling her Little Monsters to do their homework and appreciate American popular music’s rich history.

One of their last, and most bittersweet, moments of mutual respect came during that 2021 Radio City show, forever immortalized in a “60 Minutes” clip that has been making the rounds on social media on Friday. After weeks of calling her “sweetheart,” the name finally came back to him when they were — where else? — onstage. “Whoa!” Bennett cried, to his duet partner’s obvious delight. “Lady Gaga!”