Demi Lovato and Ashlee Simpson teamed up in Los Angeles Wednesday night (Sept. 28) for an epic onstage mashup of their respective 2000s singles "La La Land" and "La La."

In a video shared by a fan on social media, the pop-rock pair belt the latter song together as the sound of the crowd cheering amplifies the performance to 10.

Simpson's performance with Lovato took place on the latter's Holy Fvck Tour, which is supposedly Lovato's final tour.

Watch below:

Simpson's performance arrives after a few years of relative silence from her on the music front. The singer-songwriter hasn't released anything or performed outside of her and husband Evan Ross' 2018 EP Ashlee + Evan.

Simpson's career took flight in the mid-2000s following the 2004 release of her successful debut album, Autobiography, which inspired several of today's popular artists, including Lovato.

In August, Lovato spoke to Zane Lowe for Apple Music 1 and shared how artists such as Simpson and Kelly Clarkson inspired them.

"I saw the transformation of her as an artist go from R&B soul pop to pop rock," Lovato said about Clarkson. "And I was like, 'Wait, that's what I want to do!' And there was also Ashlee Simpson's 'La La,' and I was like, 'I want to do that!' So I got inspired, and then I found bands like Paramore and Flyleaf."

Watch Ashlee Simpson's "La La" Music Video:

Simpson's career experienced a setback when she appeared as a musical guest on Saturday Night Live in October 2004. A technical flub revealed Simpson had been lip-syncing due to an issue with her throat at the time, resulting in Simpson performing a brief, awkward dance to cut through the tension.

However, Simpson didn't let the infamous live TV moment stop her from pursuing music. She released her successful sophomore album, I Am Me, the following year.

"It's something that happened to me, and things in life happen to you, and they make you stronger, and they make you a better performer, a better person. I think things like that build your character and your strength, and it's how you handle them," she told E! News in 2018.

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