New York ? Once, back in the 1960s, Placido Domingo and his wife, Marta, locked themselves in a Vienna hotel room for three days so he could learn his role in Verdi’s “A Masked Ball.”

Skipping sleep, the promising tenor was urged on by Marta, his muse.

“I helped him memorize,” she remembers. “There were times at night when I could not any more and I said, ‘I’m falling asleep.’ So he went into the bathroom to keep on memorizing.”

The 26-year-old Spaniard made his debut in Berlin on May 31, 1967, and went on to become a tenor among tenors, with 118 roles in his repertoire.

As for his wife, she has provided crucial support from backstage or from the audience for the past four decades, becoming “my helpmate and my best critic ever,” he says.

“I am not in the shadow, you know?” she said in a recent interview. “On the contrary this is light!”

On April 1, Marta Domingo’s light will grow brighter. She’ll make her own Metropolitan Opera debut, directing her husband in a new production of Ermanno Wolf-Ferrari’s “Sly,” an opera inspired by Shakespeare’s “The Taming of the Shrew.”

‘We have a destiny’

As Marta Ornelas, she was a star soprano before Placido Domingo could call his singing a career. Already in 1962, she was considered the best interpreter of Mozart in her native Mexico after singing the role of Susanna in “The Marriage of Figaro.”

It was in Mexico that she first saw her future husband, onstage. She was “twentysomething,” she says, and he was 19.

“He was so beautiful, so slim. He caught my attention but artistically. It never crossed my mind that I might fall in love with him one day and we’d get married.”

The two joined Israel’s National Opera, appearing together in productions during their two years in Tel Aviv. Soon after, she laid aside her own career to nurture her husband’s and raise their two sons.

“We believe in God, and we believe we have a destiny,” says Marta Domingo. “It’s one of those things you cannot explain.”

She has had a decade-long career as an opera stage director, starting in 1991 with Camille Saint-Saens’ “Samson et Dalila” in Puerto Rico, with her husband as the lead tenor.

“When I directed him, it was practically like directing traffic!” she jokes. “What we have done through all these years is analyze and study every character. So by the time I directed him, it was purely traffic.”

In 1999, she directed tenor Jose Carreras in the role of Sly when the production was first performed, in Washington.

“The character is not like a dress that everybody has to wear the same way,” she says. “Placido has a sense of humor, he’s happy by nature. So to this tragic character an alcoholic, self-destructive person he adds this little touch of mischief and humor. With Placido, the moments of sunshine come out.”

Rehearsing together at the Met, she says, “we keep on analyzing the score, and analyzing.” And the work continues at home, in their East Side apartment.

“We change opinions. We change points of view.”

But they never argue, she says.

Onstage at the Met, “I say to him, would you please lie down here, and he’ll say I’d rather lie down on my left, not my right. He has to be comfortable to deliver better.”

Offering encouragement

It was Placido who encouraged her to direct the production. “And now, at rehearsal, he often gives me a thumbs up, like Caesar would give to the Roman gladiators he liked.”

And she encourages him, as well. Take one Saturday rehearsal last year of Verdi’s Requiem, which Placido was conducting at Constitution Hall with the Washington Opera Orchestra and Chorus and four soloists. It was the last rehearsal before a live national radio broadcast.

“Let me have your eyes so I can breathe with you,” he told the chorus, whispering the opening words of Verdi’s Mass for the dead, “Requiem aeternam, dona eis Domine,” meaning, “Eternal rest, grant them Lord.”

With his sleeves rolled up, glasses perched on his nose, Domingo started conducting, drawing together the force of about 200 singers and players.

Marta Domingo sat in the front row, her right hand moving in sync with her husband’s.

After the piece’s apocalyptic climax, Placido turned and glanced at Marta. She nodded. It was OK.

At 61, an age when most opera stars have retired or faded, Placido Domingo still is the world’s top box-office draw as a tenor, while building other careers as conductor and artistic director of both the Los Angeles and Washington opera companies.

His refined timbre as a singer comes after a period, around the time he turned 50, when a certain strain could be heard in his high notes.

No more. Domingo’s voice is less sweet than in his youth, but the darker, virile power that came with age has enhanced his artistry. He has long bypassed tenor Enrico Caruso’s record for most Met opening nights it’ll be 20 coming up in September (Caruso sang 17.)

The bathroom of the Domingos’ apartment is filled with wilted bouquets that Marta has saved from Placido’s Met appearances; long-stemmed roses dry, upside down, on the shower rod.

Deutsche Grammophon recently issued a four-CD set of every tenor aria Verdi ever wrote, as sung by Domingo often with Marta Domingo as unaccredited coach.