"But when it comes to piano-and-vocal singles, there's only ever been three of them - this one, Adele's Someone Like You and Bruno Mars' When I Was Your Man.
"The biggest thing for the song was my Grammy performance," he says of his spellbinding rendition of the song at January's televised awards ceremony in Los Angeles. "It took it from here to there." He smiles. "You never know what's gonna propel these things."
Legend was moved to co-write All of Me by one of his managers. "She's an old friend of mine and she said I should write a song with a similar message as Billy Joel's She's Always a Woman. She knew my relationship with my then-future wife, so she thought I should write a song like that to Chrissy."
Chrissy is Chrissy Teigen, a model who Legend met on the set of the video for his 2007 single Stereo. They were married last year, and the video for All of Me - featuring Legend and Teigen - incorporates footage from their Lake Como ceremony.
Little wonder that All of Me is already a wedding standard and piano-bar fixture.
Love in the Future was released in August last year. Factoring in its bonus tracks, it's a towering, 20-track monument to the songwriterly craft of modern R&B, melding the vintage feel of Donny Hathaway with the cutting-edge sonic nous of executive producer Kanye West. West and Legend are friends and peers.
What do people get wrong about the temperamental rapper and fashion-plate? Legend gives the question some thought before answering.
"Well, I mean, he says what he says," which is one way of describing West's penchant for the provocative and the egomaniacal.
"People see negativity coming from him sometimes, but what I see is his true love for art and music. And that makes him one of the most important and prolific and creative artists we have in the business."
And of course All of Me was the song Legend played at the nuptials of West and Kim Kardashian. While Legend has been well-known for a long time, he admits that All of Me has taken his fame to another level.
"This is the first song I've had that's truly been a huge crossover hit," Legend says appreciatively.
"It's just exposed me to fans who don't listen to R&B radio."
He pauses and smiles again. "Frankly, more white people!"
Live preview
Who: John Legend
Where and when: Vector Arena tomorrow night.
