By Bill Trott

(Reuters) -Tony Bennett, the smooth American singer who had an enduring hit with "I Left My Heart in San Francisco" and remained perpetually cool enough to win over younger generations of fans well into the 21st century died on Friday, his publicist said.

Bennett was 96. He died at his home in New York City of age-related causes, his publicist Sylvia Weiner said in a statement.

No less than Frank Sinatra called the former singing waiter "the best singer in the business" after he became a star in the 1950s. Bennett went on to win 20 Grammy awards, including a lifetime achievement award.

The older he grew, the more diverse his collaborators became. Bennett was in his late 80s when he recorded a 2014 album of duets with the outre Lady Gaga and went on a world tour with her in 2015. Partners on his popular "Duet" albums ranged from former Beatle Paul McCartney and soul queen Aretha Franklin to country star Willie Nelson and U2's Bono.

Bennett marked his 90th birthday in 2016 with a party in New York that drew celebrities such as Bruce Willis and John Travolta. The Empire State Building put on a light show in his honor. He also published a memoir in 2016 titled "Just Getting Started."

Bennett revealed in early 2021 that he was diagnosed with Alzheimer's Disease in 2016, but he kept recording after the diagnosis and later tweeted, "Life is a gift - even with Alzheimer's."

Due to his illness, Bennett retired from performing after his final concert at Radio City Music Hall On August 3 and 5, 2021.

Bennett's career was filled with highs and lows.

He was in his 50s in the late 1970s when he found himself facing a decaying marriage, a cocaine habit, a $2 million tax debt and limited career prospects. He pulled out of it by turning over his management to his son Danny, who propelled his father to new heights of popularity by introducing him to younger generations.

DISCOVERED BY BOB HOPE

Before that, Bennett had been one of the most popular singers of the 1950s - thanks to his discovery by comedian Bob Hope - until the rise of rock 'n' roll undermined him. He rebounded from that by aiming at a more mature audience.

Through it all, Bennett maintained a cool, smiling demeanor, tried to stay faithful to the material he loved best. He always thought of himself as a jazz singer.

Anthony Dominick Benedetto was born Aug. 3, 1926, in New York City. He was only 10 when his father died and his mother struggled as a dressmaker to support him. As a boy, his love of music was matched only by his interest in painting. He would be a serious painter throughout his life and sold his works under his given name.

After serving as an infantryman in Europe during World War Two, Bennett was singing under the name Joe Bari when Hope caught his act in New York's Greenwich Village. The comedian was so impressed that he had the singer change his name to Tony Bennett and used him as an opening act.

Bennett signed with Columbia Records and the result was a string of pop hits such as "Because of You," a cover of the Hank Williams country standard "Cold, Cold Heart," "Blue Velvet" and "Rags to Riches." Legions of screaming teenage girls packed his shows.

As the rock era began in the mid-1950s, Bennett moved away from pop songs toward jazz, working with some of the top names in that genre and recording "Basie Swings, Bennett Sings" with the Count Basie Orchestra.

He pulled his material from jazz and the works of writers like Cole Porter, Johnny Mercer, George and Ira Gershwin and Richard Rodgers and Lorenz Hart.

LOST HEART

In the next phase of his career, Bennett recorded "I Left My Heart in San Francisco" in 1962 - a tune by two little-known songwriters that his musical director, pianist Ralph Sharon, had stashed away. It reached only No. 19 on the Billboard chart but became his signature song.

"People ask me, 'Don't you get tired of singing that song about San Francisco?'" Bennett said in a Reuters interview. "I say, 'Do you get tired of making love?'"

In 2016 a statue of Bennett was unveiled outside San Francisco's Fairmont Hotel, where Bennett first performed the song some 55 years before.

When Danny Bennett revived his father's career in the late '70s, the singer reunited with Sharon and his 1986 album "The Art of Excellence" became his first chart album in 14 years. Through Danny's marketing, he was discovered by a youthful audience that found Bennett cool and he appeared often on the youth-oriented MTV television network. His "MTV Unplugged" album won the top Grammy as album of the year in 1995, as well as best traditional pop vocal performance.

"Tony Bennett has not just bridged the generation gap, he has demolished it," the New York Times wrote in 1994. "He has solidly connected with a younger crowd weaned on rock. And there have been no compromises."

His two "Duets" albums in 2006 and 2011 were hits and brought him wide appreciation among younger listeners because of his collaborations with younger stars.

They also appealed to millions of young people with old classics such as "Stranger in Paradise," "The Way You Look Tonight," "Rags to Riches," "I Wanna Be Around," "The Lady Is a Tramp" and "Body and Soul."

A third album of duets - this one with stars of Latin music - was released in 2012 and he recorded an album with Lady Gaga in 2014.

In June 2007 Bennett married former teacher Susan Crow after an 18-year relationship. He had four children with his two previous wives, Patricia Beech and Sandra Grant.

(Writing and reporting by Bill Trott; additional reporting by Brendan O'Brien in Chicago; Editing by Diane Craft, Doina Chiacu and Jonathan Oatis)