When you're a relatively unknown band, you can only dream of performing live on television for millions alongside one of the world's biggest pop stars. That dream became a reality for Ian Axel and Chad Vaccarino of A Great Big World, who received the rare opportunity to open for "The Voice" last Tuesday night, performing a duet with coach Christina Aguilera.

"I honestly don't remember the performance. I did not want to see Cee Lo or Adam Levine or any of those guys because we hadn't met them yet," Axel says.

Luckily for them, the stripped-down piano performance of "Say Something" was an instant hit, jumping to #1 on iTunes and receiving praise from the likes of OneRepublic, who tweeted, "Love it when a great song comes outta nowhere & explodes. Kinda like this: 'Say Something' by @AGreatBigWorld."

Love it when a great song comes outta nowhere & explodes. Kinda like this: "Say Something" by @AGreatBigWorld http://t.co/RDuwvQdyP9

— OneRepublic (@OneRepublic) November 6, 2013

Although A Great Big World only formed in 2012, this isn't the New York-based duo's first time at the rodeo. Their single "This is the New Year" was covered on "Glee" earlier this year, and the band hit their stride when "Say Something" was featured on "So You Think You Can Dance." That's when someone from Aguilera's team stumbled across the song, presented it to the pop diva and the rest is history.

"We got a call like 'Are you guys open to collaborating with Christina Aguilera?' " frontman Axel recounts, "Of course we're open to collaborating."

The pair admit the final song, which was inspired by "some pretty bad heartbreak," turned out completely different than what they expected -- in the best way possible. That's because Aguilera came in with "beautiful melodies" already crafted.

"We learned where she is in her life," Vaccarino says of working with Xtina. "And that she only wants to take on projects that inspire her," Axel added after the pair revealed it was the "Voice" coach's idea for them to perform on the major stage.

It seems Aguilera is truly inspired, posting an instagram with the humble pair from the set of their music video.

"It looks better than real life," Axel says of their first big-budget video, one they still call "simple" and plan to release as soon as possible. And while they're still reeling from their newfound success, A Great Big World were also announced on Tuesday (November 12) as performers at the 2013 Victoria Secret's Fashion Show alongside Taylor Swift and Fall Out Boy.

As for their exes who inspired the hit song, the pair have moved on. "You can't take credit for those songs," Vaccarino says, before Ian adds, "Yes, absolutely. Amen."