Madonna & Kylie Minogue's Surprise Duet That Was DECADES in the Making!

Madonna had a special surprise in store for the third of five sold-out Celebration tour shows at L.A.'s Kia Forum Thursday night... a duet with Australian diva Kylie Minogue!

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Madonna and Kylie, riding high on the success of her "Padam Padam" single, have long been compared and contrasted as queens of pop, but have always had kind words to say about each other. Last year, when Madonna was recovering from a life-threatening bacterial infection, Minogue told E! she hoped Madonna would have a speedy recovery.

She went on to say, “I feel for her. I feel for everyone touring, but especially women touring. I think it’s just that bit more work for us and it is so taxing on you as a whole. I will definitely be getting my ticket and going to see Madonna when she gets back on the road.”

Presumably, Kylie did not need a ticket! She arrived early for the show and watched most of it from the audience, where she mingled with ex-flame Lenny Kravitz, a longtime Madonna collaborator.

Fans assumed Madonna would ask Kylie to be her guest judge for the night as part of a vogueing ball sketch, but Emmy winner Ali Wong was tapped for that slot, camping and vamping it up with the superstar as Madonna's dancers vogued across the catwalk.

Just as fans were beginning to lose hope, Madonna graciously welcomed Minogue to the stage for an acoustic segment. The ambitious blondes — never before photographed together — warmly embraced.

"Now, this is what I call a survivor, okay?" Madonna, 65, said of cancer survivor Minogue, 55. "You are a fighter, too. God bless you. Never give up. You never did."

Kylie replied by gushing, "I can tell you I feel uh-mazing. This moment has been a long, long, long, long time coming."

Madonna joked, "I know — why did it take so long?"

After commanding the crowd to turn their phones' lights on — "Everyone's too busy taking pictures. They're gagged!" — Madonna led the two in a soulful and particularly meaningful cover of Gloria Gaynor's "I Will Survive."

As Madonna sang, "I could never live without you by my side," Minogue placed her hand on her shoulder and the two shared sweet smiles.

As a cherry on the top of the love fest, Kylie was sporting a glittery "MADONNA" tank that referenced a "KYLIE MINOGUE" tank Madonna wore at the 2000 MTV Europe Music Awards.

After their chemistry-filled duet on the tune (punctuated by Madonna playfully branding the audience "motherf**kers!"), Madonna asked, "Let's see if they know this song." Then, the two burst into part of Kylie's Top 10 U.S. hot "Can't Get You Out of My Head."

This rendition included a little grinding from Madonna, jumping from Kylie, and a final and extended hug. Kylie danced off as Madonna's next number, "La Isla Bonita," cued up.

Reaction online was swift and fawning, with fans shocked that the long-hoped-for meet-up came to fruition, and leading some to start guessing who might be in store for Madonna's next two L.A. shows. A long list of possibilities was bandied about, everyone from Beyoncé to Lady Gagay, Michelle Visage or another big personality from "RuPaul's Drag Race."

After the show, Minogue modeled her "MADONNA" shirt on Instagram, writing, "What the tee says ..." Thanking Madonna, she posted video of herself dancing wildly to "Ray of Light," writing, "MADONNA 💓 It’s been a long time coming!!! LOVED being with you!!!! Celebration Tour AND it is now International Women’s Day …. THANK YOU and LOVE LOVE LOVE 😘😘😘."

As Madonna continues her tour, ending officially in Mexico City on April 24 (with some reports suggesting a one-off gig in Brazil in early May), Kylie will continue her first-ever U.S. residency in Las Vegas, at the Venetian’s Voltaire, through May 4.