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Lady Gaga, Elton John

Dimitrios Kambouris/WireImage

Elton John delighted scores of fans Saturday afternoon when he gave a surprise concert at a legendary music site in West Hollywood, where he was also joined by a surprise guest.

Sporting a midnight blue buttoned military-style mini dress, sky high platform lace-up boots and shoulder-length platinum blond hair, Lady Gaga took the stage during his set and took on George Michael's former role, performing with John, who sported a black and red embroidered jacket and red shirt, a duet of his hit "Don't Let the Sun Go Down on Me."

"Mad about the girl," John wrote on Instagram.

The concert was livestreamed on AOL. Lady Gaga and John performed in the parking lot of the former Tower Records building, a popular music landmark on the Sunset Strip for decades. Gibson Guitars now owns the space and the record store's sign was recently repainted amid the release of Colin Hanks' documentary All Things Must Pass: The Rise and Fall of Tower Records, which hit theaters in October.

Also spotted at the concert—Lady Gaga's fiancé, Taylor Kinney. The actor was spotted heading out of the VIP area and was followed by the singer, who greeted fans and took some selfies, E! News has learned.

John and Lady Gaga are longtime friends and have collaborated on music before—they sang the duet "Hello, Hello" for the 2011 animated movie Gnomeo & Juliet. Also, she is the godmother of his and husband David Furnish's two sons, Zachary, 5, and Elijah, 3.

"She buys them presents on their birthdays and at Christmas, but more than anything, which is the best thing, she's a good godmother," John said on The Talk Friday.

"She comes and she bathes them, and she sings to them and she tells them stories, which is the best thing of all," he added. "It's the personal thing. To do something and to give your time...it's easy to buy a present and send it round. She doesn't just do that, she comes round and spends time with the children and that's what I love about her."

Teacup madness @davidfurnish @disneyland #ShareTheLove

A photo posted by Elton John (@eltonjohn) on

On Sunday, Lady Gaga, 29, is also set to perform at the 2016 Oscars, which will make her the first artist to play the Academy Awards, the Grammys and the Super Bowl in a single year. She performed a David Bowie medley during a tribute performance for the late rocker at the Grammy Awards, sporting bright red hair like his Ziggy Stardust alter ego, and sang the national anthem at the NFL championship game.

At the Oscars, she will sing her song "Til It Happens to You," which is nominated for Best Original Song, marking her first Oscar nomination. The track is featured in the Oscar-nominated campus rape documentary, The Hunting GroundVice President Joe Biden will introduce her onstage and also speak out about sexual assault awareness.

Also on Oscar Sunday, John, 68, will host his annual Elton John AIDS Foundation Academy Awards Viewing Party.

"We shut down Sunset!!!" he said on Instagram. "Thank you from the bottom of my heart to everyone who supported today's surprise gig. Massive thanks to the City of West Hollywood for supporting the Elton John AIDS Foundation."