Sam Smith was by some measures last year's biggest breakthrough musician, but on his latest song, he is sadder than ever.

Smith returns to his familiar subject matter of fleeting love on "Drowning Shadows," a previously unreleased song that will appear on a deluxe edition of his blockbuster album "In The Lonely Hour" that comes out Friday.

"Brace yourself, this is going to be really depressing. So sorry if anyone's in a happy mood," Smith joked as he premiered the song Monday on Apple's Beats One radio.

Described by Smith as "the saddest song I've ever written," the 23-year-old English crooner demonstrates the power of his soaring, soulful voice on "Drowning Shadows" as he narrates his battles with alcohol and casual sex.

Smith sings: "I don't know how I've reached this place / So far from heaven, so far from grace.

"Go home to nothing or stay out more? Give in to someone or lock down my door? Or drown all my shadows, drown them like before?" he sings to piano and strings.

Smith explained to radio listeners that the song was in fact written prior to "In The Lonely Hour" but was left off the album because "something didn't work" in the production.

The singer, who re-recorded the song, said that "Drowning Shadows" was about his former life as he returned home each evening in London from the studio in a taxi.

He would face a daily crossroads as he decided whether to hit the gay bars or go home alone.

"I was in such a lonely, sad place when I was recording this album," said Smith, who was a bartender until a few years ago.

"I used to go out by myself and drink and try and find love. Or I would just go home and chill," he said.

But the album proved to be a global hit backed by the strength of the ballad "Stay With Me," itself a tender reflection on the aftermath of a one-night stand.

Smith has since gone on to pack stadiums and won three of the four most prestigious awards at the latest Grammys.

More recently Smith, who successfully underwent surgery for vocal cord problems, composed the theme song for the latest James Bond film.