As the mastermind behind this year's addictive sing-a-long tune, Happy, Pharrell Williams is the last celeb you'd expect to cry.

But that's exactly what happened during the singer's interview with Oprah Winfrey.

Pharrell - and his equally famous hat - joined the legendary talk-show host for an episode of Oprah Prime, whichaired in the US last night.

During their chat, Pharrell explained how the success of the first single from his new album, G I R L, came from nowhere.

"Zero air-play, nothing. And the next thing you know, we put out the video on 21 November and all of a sudden - boom. I mean, when I say boom, I mean 'booooom'", he explained.

"And we were like, what's happening? First of all, people are putting up their own videos. It was like no longer my song."

Oprah then played a montage of fan videos to the song - and the hit-maker got overwrought with emotion.

Wiping tears from his eyes, he joked: "Why am I crying on Oprah?

"It's overwhelming because it's like I love what I do", he went on to explain.

"I just appreciate the fact that people have believed in me for so long, that I could make it to this point to feel that."

Oprah praised the musician via social media as the episode aired.

"@Pharrell this was one of the most sincere humbling moments I've ever experienced during an interview. HAPPY tears. #OprahPrime", she tweeted.

The Get Lucky singer responded: "Wow... Oprah I am speechless. All of your kind words are overwhelming".

Wow...@Oprah, I am speechless. All of your kind words are overwhelming.

— Pharrell Williams (@Pharrell) April 14, 2014