The One Direction singer reveals he was in Sin City last year (16) to attend one of Calvin's concerts, but when the Scot cancelled, Steve stepped in as a last-minute replacement.

"I was on holiday with my friends in Vegas and we were supposed to see a Calvin Harris show," the Brit tells Rolling Stone. "Calvin pulled out and then Steve did the show and we had an incredible night. It was really, really fun. The show was amazing."

Louis decided to reach out to the Delirious hitmaker and propose hitting the studio together for his first solo offering since One Direction embarked on its hiatus in early 2016 - and he is thankful he did.

"I messaged Steve after the show and said, you know, I was a big fan and I really enjoyed the show, and maybe we should do something together, kinda just on a whim," he smiles.

The result was the upbeat Just Hold On, but Louis admits the tune actually began as a ballad.

He recalls, "When the song started out, it was kind of slow, emotional, sentimental ballad, that's not really the concept that we wanted for the song..."

The duo decided to speed it up to appeal to the masses, but the artists were determined not to lose the message of hope behind the uplifting track.

Aoki confesses, "It's the most human song I've produced."

Louis debuted the single with Aoki in a performance on British reality show The X Factor in December (16), just three days after losing his mother to cancer.

Johannah Deakin was just 43 at the time of her death, and Louis recently opened up about her passing for the first time in a U.S. radio interview on Sirius XM, revealing he pressed on with his music in memory of his mum.

"When I first found out the news I kind of did want to throw the towel in," he confessed. "But it was my mum who said to me that I've just got to keep going, she told me very sternly that she wanted me to."

He added of performing so soon after his family tragedy, "It was tough, but I felt like it was nice for me to almost have a little send off for her."