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Madonna, Taylor Swift

Kevin Winter/Getty Images for iHeartMedia

In case you didn't tune into the iHeartRadio awards last night, here's a little spoiler: Taylor Swift and Madonna stole the show when the two pop icons performed a surprise duet. 

During the 56-year-old singer's performance of "Ghosttown," the four-time Grammy winner joined the Queen of Pop onstage to provide the acoustic guitar while the iconic musicians, who have a 31-year age difference, both rocked matching lace thigh-highs. 

So, how did the collaboration between the two come to be? 

A source tells E! News that their friendship began awhile back after Madonna applauded Taylor's songwriting skills in interviews. Swift, who has been a longtime fan of Madge, of course took the compliments to heart and has also gushed over the "Living for Love" beauty. 

VIDEO: Taylor Swift joins Madonna for surprise duet

Madonna, Taylor Swift, iHeart Radio Awards

Kevin Mazur/Getty Images for iHeartMedia

We're told their collaboration happened organically, and it was simply a natural extension of their "their lovefest." 

Meanwhile, a second source tells E! News that Swift and Madonna's performance was so tightly guarded that many members of their camps were unaware of any partnership until just hours before the pair took the stage. 

"Madonna wanted to shock and surprise people in the best way possible and managed to pull it off," the insider says. 

While T.Swift and Madge may seem like unlikely BFFs, the two have far more in common than one may expect and they're both big supporters of each other's careers. 

"She has an opinion, and she's going against the norm," Madonna previously told Rolling Stone of the "Black Space" singer. "So in that respect, she is similar to me, yeah. And also, people just want to give her a hard time all the time because they think she's a goody-two-shoes, so of course I want to embrace her."

READ: More of the best moments at the iHeartRadio music awards 

Taylor Swift, Madonna, Grammy Awards, Candids

Alberto E. Rodriguez/WireImage

Additionally, Taylor had a total fangirl moment back in January when she caught wind of Madonna's kind words on Australia's Today"It's good to have princesses [of pop]. It means there's lots of pretty dresses around. I like Taylor Swift. I think she writes some really catchy pop songs," the Material Girl revealed. "I can't get them out of my head."

Swift responded to the praise via Tumblr, writing, "Thanks now I'm dead," in addition to tagging the post with the hashtags "#how am i supposed to deal with this" and "#BE COOL TAYLOR STOP BEING EMBARRASSING."

"One element of Madonna's career that really takes center stage is how many times she's reinvented herself," Swift previously told Billboard of the pop icon. "It's easier to stay in one look, one comfort zone, one musical style. It's inspiring to see someone whose only predictable quality is being unpredictable."

—Reporting by Alicia Quarles

PHOTOS: All the stars attending the 2015 iHeartRadio Music Awards