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Plácido Domingo's personal motto, "If I rest, I rust," says it all.

His refusal to cut back or slow down, after an astonishing career spanning 56 years, tells you what you need to know about the industriousness and fortitude of opera's marathon man.

It also speaks to his innate love of performing, an all-encompassing need to put his star presence out there in such a way as to make the classical superstar whom James Levine, former music director of the Metropolitan Opera, once called "a one-man golden age of opera" one of the top continuing attractions in opera.

"Placido is a force of nature," says conductor James Conlon, who has known and worked with Domingo for nearly 40 years and, as music director of L.A. Opera, has addressed him as "boss" for the past decade. "There is nobody like him today — he is unique. What's amazing is not just how well he does at everything, but also how long he has done it and how well he keeps going."

Indeed, few opera singers make it into their 70s with voices that are in as solid a shape as Domingo's. Fewer still have come anywhere close to the singer — who turned 76 in January — in sheer enormity of repertory, performances and recordings, not to mention breadth of professional endeavor.

He also conducts opera and concerts all over the world, serves as general director of Los Angeles Opera (since 2003) and administers Operalia, an annual international competition to foster the next generation of classical vocal talent.

Beginning with performances in the title role of Verdi's "Simon Boccanegra" in 2009, one of the greatest operatic tenors of all time reinvented himself as a baritone. Since then, Domingo has been taking on some of the most formidable lyric baritone roles in Italian opera. Such roles have given him new musical worlds to conquer, while extending his distinguished career on the operatic stage.

Domingo's priorities lie with the opera companies that represent his home base in the U.S. — Los Angeles and the Met, where he has sung some 700 performances and conducted more than 150 times.

So what keeps Domingo going at such a hectic pace, season after season?

"It's the great, great passion that I feel for making music," he said. "The doctors say I am fine. That's the important thing. I know the voice is healthy, and I am still in love with being on the stage. As long as the public comes to see me and I feel fine, I will continue."

Mr. Nice Guy

Many of Domingo's colleagues have remarked about the absence of vainglory in his personality — the Mr. Nice Guy persona that sets him apart from so many temperamental stars at the top of the classical heap. (After all, his name translates as "peaceful Sunday" in Spanish.)

"In all the years of our working together, we have never had an argument," said Conlon. "Working with Placido is easy because he's so smart and he's done everything. He makes it so clear what he needs and what he wants that we communicate through a sort of mutual osmosis. Here at L.A. Opera, it never feels like he is the Boss."

Born in Spain and raised in Mexico, Domingo inherited his fierce work ethic from his parents, singers who founded and ran a zarzuela troupe that presented the Spanish form of operetta throughout Mexico. ("I think the theater was in my veins already," he recalled.) He graduated to voice lessons after early studies in piano and conducting. He played piano in bars and took bit parts in TV shows before his breakthrough as an opera singer in his early 20s. He made his operatic debut in Monterrey, Mexico, in 1961, as Alfredo in "La Traviata."

Domingo already had a flourishing international career of more than 30 years before the launch of the Three Tenors franchise on the eve of the 1990 World Cup final in Rome. What was intended to be a one-off concert turned the supernova trio of Domingo and colleagues Luciano Pavarotti and Jose Carreras into a wildly successful global phenomenon. These stadium gigs became the most lucrative venture ever undertaken by any classical artist, grossing many millions of dollars in ticket and album sales, and reaching billions of TV viewers throughout the 1990s.

Wife plays key role

Domingo credits Marta Ornelas Domingo, his wife of 55 years and a former singer turned stage director, with playing a key role in his shaping his "artistically meaningful" Indian-summer career. The couple have three children and eight grandchildren, and they have worked together on numerous opera productions.

He cannot pretend he hasn't suffered health problems in recent years — including a bout with colon cancer, a pulmonary embolism and the removal of his gallbladder. But, he said, he currently is "feeling better" than he has in the past five years and doesn't take "crazy risks" with his health. His virtually cancellation-free track record attests to his tenacity.

He has contracts to sing and conduct for the next three years, he noted, and recently extended his contract as head of L.A. Opera through 2021-22. That doesn't sound like a man on the verge of packing it in. Indeed, the joys others find in retirement cannot compare to the joys Domingo still finds in performing and being active in music, he said.

"I have no idea how long my voice will last, but so far I haven't felt the need to slow down. When I reach the point where it becomes too much physically, I will stop singing opera, then I will give more concerts."