It wasn’t the first time he’d heard her sing. But looking back, it was Latvian soprano Kristine Opolais’s 2004 performance of Tatyana’s “Letter Scene” from Tchaikovsky’s “Eugene Onegin” that truly electrified the young conductor Andris Nelsons.

“I thought — oh my God . . . It’s not only the music,” said Nelsons, who will take the baton when Opolais reprises Tchaikovsky’s opus with the Boston Symphony Orchestra from April 21 through 23. “When I saw her doing Tatyana is when I got really nights not sleeping — in love.”

The revelation was no less staggering for Opolais. “I was thrilled by him as an extremely talented conductor,” she recalled, then she found herself drawn to him in other ways.

At the time, Nelsons was music director of the Latvian National Opera, where Opolais began in the chorus before rising to soloist. And he was married. “It was something really uncomfortable what I felt,” she recalled, “because I understood with the other side of my brain that it was impossible.”

Wed in 2011, Nelsons and Opolais are today classical music’s hottest couple: He holding prestigious appointments in Boston and (soon) Leipzig; she fresh off a lengthy engagement at New York’s Metropolitan Opera, winning acclaim in the title roles of Puccini’s “Madama Butterfly” and “Manon Lescaut.”

The performances were beamed to hundreds of thousands via the Met’s Live in HD program, with The New York Times saying Opolais “sounded as glamorous as she looked.”

Read the entire feature via the Boston Globe.