SAN JUAN, Puerto Rico — Driving through the Condado district of Puerto Rico’s capital city, Justino Díaz squints a little as the sun pecks at us from between the high-rises lining the beach.

San Juan wasn’t always this shadowy, he tells me. We roll past the hospital where he was born in 1940, past the Robinson School he attended as a boy and the adjacent chapel where, at the age of 8, he sang for the very first time in public (well, an assembly of the PTA).

Díaz may not be a household name on the level of his fellow Kennedy Center honorees Berry Gordy, Bette Midler, Lorne Michaels or Joni Mitchell — opera is only allotted a handful of those per era — but the bass-baritone’s rise from humble beginnings in Puerto Rico to the world’s greatest stages is the stuff of opera legend. Díaz’s 40-year career as a performer and his decades of advocacy and engagement with the art form make him an undersung hero of American opera. Though it’s hard to imagine him undersinging anything.

He’s also got a deep connection to D.C., originating the role of the fearsome Francesco in the world premiere of Alberto Ginastera’s “Beatrix Cenci,” a commission made as part of the 1971 inauguration of the Kennedy Center (back when what is now the Washington National Opera was known as the Opera Society of Washington). Díaz made several appearances with the WNO in the decades that followed, including as the challenging lead in Verdi’s “Attila” in 1976 and, most recently in 2000, revisiting “Otello” and “Il Trovatore.”

Díaz now lives in one of those high-rises on Condado Beach with his wife, Ilsa Rodriguez, a retired flight attendant. He has two daughters from a previous marriage, actress Natascia Díaz and composer and singer Katya Díaz. From 15 floors up, you can see the long ribbon of the beach dotted with slowly returning tourists, and a vast cerulean stretch of sea biting a steep shelf into the shore.

The walls of Díaz’s study are overtaken with posters. It’s a gallery of casually accumulated evidence of his colossal career: as Maometto in “L’Assedio di Corinto” (opposite Beverly Sills as Pamira) at La Scala in 1969 and the Met in 1975; as Scarpia in “Tosca” at the Staatsoper in 1972; a framed 1988 New York Daily News clip hailing Díaz as “the finest Met Iago since Tito Gobbi.”

(Reviewing Díaz’s Scarpia at the Kennedy Center that year for the Washington classical radio station WGMS, former Washington Post music editor Paul Hume echoed this sentiment, writing that “for subtle thrusts combined with sheer open animal brutality, he stands easily alongside the greatest protagonist I’ve ever heard in the part, who was, of course, Tito Gobbi.”)

And then there’s the poster for the 1966 inaugural performance at the Metropolitan Opera House at Lincoln Center, a production of Samuel Barber’s “Antony and Cleopatra” adapted by Franco Zeffirelli, in which Díaz played the lead opposite the legendary Leontyne Price. It launched Díaz’s career, as well as a long artistic relationship with Zeffirelli, including a 1984 production of “Carmen” and the 1986 cinematic adaptation of Verdi’s “Otello” — a visually arresting showcase for his flawless Iago. A model of the Sphinx the director made for the set of “Antony” now perches atop Díaz’s bookcases, overlooking a portrait of a regal-looking Díaz in costume as King Phillip II (from Verdi’s “Don Carlo”).

Another hall is entirely posters marking various editions of the Casals Festival — a storied classical music event started in 1956 by the Spanish Puerto Rican cellist, composer and conductor Pablo Casals. The festival has brought some of the biggest names in music to the island, from Mstislav Rostropovich and Leonard Bernstein to Eugene Ormandy and Zubin Mehta. Díaz and pianist Elías López Sobá served as artistic directors between 2003 and 2009.

Music made an impression on Díaz as far back as he can remember. After World War II, his father purchased a Hallicrafters radio from the Sears, Roebuck and Co. catalogue, as well as a phonograph for his ever-growing collection of classical and opera music. On the radio, Díaz listened to the serials “Dick Tracy,” “Superman” and “The Lone Ranger,” the theme to which was his introduction to Rossini.

But he was also enraptured by the sounds of his father’s recordings of Andrés Segovia, Vladimir Horowitz and Arthur Rubinstein that would fill the house on Sundays. He remembers hearing Schumann’s “Rhenish” Symphony (No. 3) drifting through the open windows from the neighbors’ house as he lay in his bed under the mosquito net. And he recalls the 78 of the “Toreador” from Bizet’s “Carmen” — “sung by Leonard Warren, thank you very much” — that his father kept in heavy rotation.

“Who could have known that would be the role I sang the most during my career?” he says.

As a teenager, he started taking voice lessons and singing in choirs in San Juan. At 17, he sang the role of Ben in Gian Carlo Menotti’s short opera “The Telephone.”

“I thought, if I’m good enough to sing in the chorus in Puerto Rico, I’m good enough to maybe have a career,” he says. “In the United States they have big opera companies, and they’re always going to need people in the chorus. I rationalized it that I don’t care if I ever make it as a soloist. I just wanted to be in music and do music and be affected by music for the rest of my life. It was a very spiritual feeling to me.”

Díaz ended up attending New England Conservatory in Boston, where, decades later, in 1986, he would receive an honorary doctorate alongside Miles Davis, violinist Joseph Silverstein and composer Virgil Thomson. He studied with Boris Goldovsky, the director of the NEC opera program and the head of the touring New England Opera Theater company. With Goldovsky’s tacit encouragement, Díaz quit the former to join the latter, the whole company piling into a bus for a string of concerts and as much as 300 miles a day on the road.

Goldovsky also assisted Díaz in connecting with the powerful music agent Hans J. Hoffmann. Díaz recalls going to Hoffman’s office at the age of 21 and singing for him. Hoffman excused himself from the room halfway through his second aria and returned with a stack of papers for Díaz to sign. Audition over.

Díaz moved to New York and started singing in the Metropolitan Opera Studio, performing student matinees and miniaturized versions of “Così fan tutte” and “The Barber of Seville.” He was starting to feel at home, but he was also lost among legends of the art form.

“If you went every night to the Metropolitan, one night you would get [Renata] Tebaldi, another night you’d get [Maria] Callas, another night you’d get [Joan] Sutherland, another night you’d get [Elisabeth] Schwarzkopf,” he says, his eyes widening at the memory. “I mean, ‘Hello?’”

In 1963, after some prodding from renowned conductor George Schick, Díaz applied for the Metropolitan Opera National Council Auditions, his trepidations balanced by an assurance that he fit a particular bill. Legendary basses such as Louis Sgarro, Norman Scott and Clifford Harvuot were all getting along in years, and if Díaz wasn’t exactly ready, he was at least prepared. He took first prize, winning a year’s contract.

He made his debut with the Met that year as Monterone in Verdi’s “Rigoletto.” Three years later, he was a leading man opposite Price.

Díaz credits the particular popularity of the roles for which he’s most famous — Escamillo, Iago, Oroveso — in part to the fortune of striking a natural physical fit (“black hair, tall, dark, handsome — perception, whatever …” he says with a weary sigh). But watch him in action in Herbert von Karajan’s 1967 cinematic adaptation of “Carmen,” starring mezzo-soprano Grace Bumbry in the title role, and there’s really nothing about Díaz that’s not Escamillo.

Bumbry, 84, has known Díaz since her 1965 Met debut, when he sang the role of the Grand Inquisitor to her Princess Eboli in Verdi’s “Don Carlo.” Bumbry, who was a Kennedy Center honoree in 2009, sees Díaz as a perfect choice for the recognition.

“It’s a wonderful, wonderful idea to give this honor to Justino,” she says by phone from her home in Vienna, “because he certainly deserves it.”

She sings the praises of his bass-baritone as “extraordinarily beautiful” and “even from the top to the bottom,” and suggests he could have also gotten away with singing tenor. She fondly remembers a night at Díaz’s house when she, Plácido Domingo, Martina Arroyo and Mehta had come to San Juan to give a performance of Verdi’s “Requiem” as part of the 1972 Casals Festival. “Díaz and Domingo were fooling around and vying against each other to see who could sing the highest,” she says with a laugh, “and the bass [i.e., Díaz] won the prize!”

Bumbry also recalls the level of detail Díaz brought to his performances — his movements, his timing, his facial expressions, the ease with which he manipulated the muleta as the toreador Escamillo.

“In all of the operas that we did, whether it was ‘Carmen’ or ‘Nabucco’ or ‘Macbeth,’ the element I found most interesting about him was [his ability to] embody the parts you’re singing.”

“He’s such a natural onstage,” says Roselín Pabón, associate emeritus director of the Orquesta Sinfónica de Puerto Rico. “He has such command of the stage — not only vocally. His acting. His knowledge of the art. His knowledge of the roles he’s performing.”

Pabón first worked with Díaz in 1982, when the singer returned home to sing the role of Oroveso in a production of “Norma” at the Centro de Bellas Artes Luis A. Ferré, a performing arts center that opened in 1981 and houses the Orquesta Sinfónica, now led by Maximiano Valdés. Since then, he and Díaz have worked together throughout their careers, with Díaz even stopping by El Conservatorio de Música de Puerto Rico to lead master classes for Pabón’s students. In San Juan in 2003, Pabón led Díaz in his final stage role as Germont in “La Traviata.”

Pabón considers Díaz the equivalent in his lifetime to the great Puerto Rican tenor Antonio Paoli, known as “the King of Tenors and the Tenor of Kings” — in the time of Caruso, no less. Like Díaz, Paoli became a worldwide sensation in his younger years, taking the stage of the greatest opera houses. Unlike Díaz, he pivoted into a boxing career when the First World War shuttered the opera world.

“He’s that star that touched the world because of his excellence,” says Pabón. “For us conductors, we have so much to worry about when we’re conducting. To have someone so secure, who sings so musically and at the same time with such ease of expression, it makes our work so much easier. And that’s the kind of singer Justino is.”

“Puerto Ricans are very proud of him,” says Ilsa, stopping by the living room on her way out. “He deserves it.” She brings up his tenacity. At 55, Díaz endured heart bypass surgery, and he has weathered both lung and colon cancer. “He has something that he loves to tell everybody,” says Ilsa. “‘I’m gonna bury you all!’ ”

“Like Khrushchev at the United Nations,” Díaz adds. “I thought it was a good joke!”

“And he will!” she says.

Though Díaz has not performed publicly since that “La Traviata” in 2003, the contours of his life are still shaped by opera, the way the surf shapes the Condado shore. He and Ilsa still eat their lunch at around 3 — a habit held over from his singing schedule. And “practically every night” he must go onstage, in dreams he calls “absurd” — in a voice that betrays some warm nostalgia.

Gentle, thoughtful and, above all, grateful, Díaz remains modest about what his career has meant to opera lovers, his fellow artists and the whole of Puerto Rico. But when it comes to what opera has meant to him, he couldn’t be prouder.

“I was just so thankful to be there, to open my mouth, to re-create the works of the great geniuses of 200 years before. That was my job. Something which Boris used to call ‘partnership in greatness.’ If I get to be a partner in greatness, what else is there?”