In 1959, when he was 18, Plácido Domingo auditioned for the National Opera in Mexico City as a baritone. The jury was impressed but told Mr. Domingo that he was really a tenor. Two years later he sang his first lead tenor role, Alfredo in Verdi’s “Traviata” in Monterrey. And so began one of the great tenor careers in opera history.

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Sara Krulwich/The New York Times

“Simon Boccanegra”: Plácido Domingo, a tenor, in the baritone role of the opera’s title character at the Metropolitan Opera House.

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Marcello Giordani as Gabriele Adorno and Adrianne Pieczonka as Maria, a k a Amelia Grimaldi.

On Monday, three days before turning 69, Mr. Domingo returned to his vocal roots. For the first time at theMetropolitan Opera he sang a baritone role, the title character in Verdi’s “Simon Boccanegra.” Some of his tougher critics would say that Mr. Domingo has been a quasi baritone for years, since he has increasingly asked conductors to transpose parts of the tenor roles he sings down a step or two.

But he sounded liberated as Boccanegra, a tormented doge in 14th-century Genoa. At times his voice had a worn cast. And when he dipped into the lower baritone register, he had to fortify his sound with chesty, sometimes leathery power. Still, this was some of his freshest singing in years.

Maybe taking on Boccanegra is a self-indulgent exercise for Mr. Domingo at this stage of his career. I almost hesitate to praise him, since I do not want him to get ideas. Right now the two companies he is running — the Los Angeles Opera and the Washington National Opera — are struggling financially. So he has big responsibilities.

That said, he earned an enormous ovation on Monday night. Over the last decade, when a role took him to the upper register of his tenor voice, he often sounded cautious and calculating. But as Boccanegra, he could not wait, it seemed, for the line to soar into the baritone’s high register, now his comfort zone.

Yet that auditioning committee of 1959 was right: Mr. Domingo was a tenor. Whether a singer is a tenor or a baritone is not just a matter of range. The coloring and character of a voice also identifies its type. There have long been dusky, baritonal qualities to Mr. Domingo’s singing, but the overall colorings and ping in his sound were those of a tenor.

Inevitably, he made Boccanegra seem like a tenor role. The long scene in which Boccanegra discovers that Maria, who goes by the name Amelia Grimaldi (don’t ask), is his long-lost illegitimate daughter, did not have the contrast of baritone and soprano colorings that Verdi intended. Still, Mr. Domingo brought vocal charisma, dramatic dignity and a lifetime of experience to his portrayal. Purists will complain, but Mr. Domingo’s performance was an intriguing experiment.

A week earlier Mr. Domingo was in the pit at the Met to conduct the first performance of Verdi’s “Stiffelio” this season, a run that continues. He did an able job. But what a difference to hear a similarly complex Verdi score withJames Levine in the pit. Mr. Levine, who conducted on Monday night, has often spoken of how much he reveres this score, and his respect came through in the somberly beautiful, nuanced playing he drew from the orchestra.

“Simon Boccanegra” is a hybrid in the Verdi canon. It was a flop at its premiere in 1857 in Venice. Almost a quarter-century later, in 1881, Verdi extensively revised the score, which combines elements of impassioned middle-period and magisterial mature Verdi.

The plot, however, is one of the most convoluted in opera. Verdi was drawn to the story because it allowed him to portray an imperfect man, once a ruthless pirate, who is conscripted into a leadership role for which he feels unfit, yet who tries to reconcile the conflicts between the plebeian commoners and the aristocracy; a man who made a mess of his personal life but eventually does right by his daughter. But do not try to untangle the strands of the plots and the multiple identities of the characters.

As Maria/Amelia, the Canadian soprano Adrianne Pieczonka was splendid, singing with clear, shimmering, pitch-perfect sound and lovely phrasing. The tenor Marcello Giordani can be a sloppy singer. But the role of Gabriele Adorno, the hotheaded aristocrat who loves Maria, suits him, and he sang with ardor and big, throbbing top notes.

The bass-baritone James Morris’s voice is weather-beaten these days. But as Fiesco, Maria’s father, he conveyed grave dignity and moving authority. Paolo, the villain (that much seems clear), was the bass-baritone Patrick Carfizzi, whose strong voice flagged as the night went on.

Giancarlo del Monaco’s tastefully grand production was introduced in 1995, when Mr. Domingo sang the tenor role of Gabriele. Even with all his drive to notch records in the opera annals, Mr. Domingo could not have imagined then that he would be singing the title role at 69.