Tony Bennett, the 94-year-old singer who has become a beloved interpreter of the American songbook, has been diagnosed with Alzheimer’s disease, his wife, Susan, told AARP The Magazine this week.

“Life is a gift — even with Alzheimer’s,” the singer tweeted on Monday morning. “Thank you to Susan and my family for their support.”

Susan Bennett, and Tony Bennett’s eldest son, Danny, told the magazine that Bennett was diagnosed with Alzheimer’s — a degenerative brain disease that causes memory loss, among other things — in 2016.

According to the magazine, Bennett began showing symptoms in 2015. “Even his increasingly rare moments of clarity and awareness reveal the depths of his debility,” the article states. But it said that he had not experienced the disorientation that prompts some patients to wander off, or episodes of terror, rage or depression.

Before the coronavirus pandemic, Bennett had continued to perform extensively. But backstage, relatives told the magazine, he could seem “mystified about his whereabouts.”

“But the moment he heard the announcer’s voice boom ‘Ladies and gentlemen — Tony Bennett!’ he would transform himself into performance mode, stride out into the spotlight, smiling and acknowledging the audience’s applause,” the piece said.

His wife, Susan, would watch nervously, worrying that he would forget a lyric. “I was a nervous frigging wreck,” she told the magazine. “Yet he always delivered!”

The early signs came in 2015, she told the magazine, when he began forgetting musicians’ names onstage, and began stashing a list on the piano, she said. But he knew something was wrong and wanted to see a doctor, she said, and he learned he had Alzheimer’s in 2016.

Susan Bennett said that he can still recognize family members, but the magazine reported that “mundane objects as familiar as a fork or a set of house keys can be utterly mysterious to him.”

Bennett, who has had a seven-decade-long career, scored his first big hit in 1951, “Because of You.” In 1962 he recorded “I Left My Heart in San Francisco,” which became his signature song. Long after other crooners had died or faded from the airwaves, Bennett experienced a resurgence in popularity: He won a Grammy for his 1994 album, “Tony Bennett: MTV Unplugged.” Since then, he has recorded duets with a string of notables including James Taylor, Sting and Amy Winehouse.

He recorded an album with Lady Gaga in 2014, “Tony Bennett & Lady Gaga: Cheek to Cheek,” which debuted at No. 1 on the Billboard’s Top 200 pop and rock chart. According to the AARP article, a follow-up album with Lady Gaga, which was recorded between 2018 and early 2020, will be released this spring.

Lady Gaga was aware of Bennett’s condition when they were recording their most recent collaboration, the article said. In documentary footage of the sessions, Bennett rarely speaks, and offers one-word responses like “Thanks” or “Yeah.”

But his appetite for all things musical remains robust. According to the magazine, he continues to rehearse a 90-minute set twice a week with his longtime pianist, Lee Musiker — and does so without any of the haltingness that can characterize his speech.

More than five million Americans are living with Alzheimer’s, according to the Alzheimer’s Association, including one in 10 people age 65 or older. Symptoms may initially include repeating questions, getting lost in a familiar place or misplacing things, and may eventually progress to hallucinations, angry outbursts, and the inability to recognize family and friends or communicate at all. Alzheimer’s has no cure.

Susan Bennett is serving as her husband’s caregiver.

“I have my moments and it gets very difficult,” she told the magazine. “It’s no fun arguing with someone who doesn’t understand you.” But she added that they felt more fortunate than many other people living with Alzheimer’s.

Bennett’s last public performance was in March at the Count Basie Center for the Arts in Red Bank, N.J. Before the coronavirus shut down live performances, he was touring often, singing a 90-minute set without cluing in audiences or critics that anything was amiss.

“He’s not the old Tony anymore,” Susan Bennett told the magazine. “But when he sings, he’s the old Tony.”