Dan + Shay were unprepared for their performance at Justin and Hailey Bieber's wedding last month, because they didn't get a chance to rehearse before they hit the stage with the groom.

The country duo, Dan Smyers and Shay Mooney, debuted their new collaboration with Bieber, titled 10,000 Hours, shortly after the newlyweds had tied the knot for the second time in a year, and though they had a "blast" at the event, Smyers has admitted nothing was planned.

"It's awesome to get to attend their wedding and it was a blast, it was a really good time. And that was like the first place we debuted the song," Smyers told Entertainment Tonight. "We had never rehearsed it!

"We obviously recorded it in the studio, but we never played it live in front of actual human beings. Justin was like, 'Sing 10,000 hours', and we're like, 'OK, we'll try.'"

"(We were) just making up the words," Mooney joked.

But guests at the ceremony didn't seem to mind the impromptu nature of the performance.

"It was good though, it was really fun and people were out on the dance floor rocking," Smyers added. "It was a good time."

The song is now a big hit among fans, entering America's top five earlier this week.