However, her tune has slightly changed now that she has come into her own as a more popular artist.
After performing in over 50 stadiums for the Reputation tour, Charli spoke to Pitchfork about how she now feels about the performances.
"I'm really grateful that [Taylor] asked me on that tour," the 27-year-old says in her cover story for the publication. "But as an artist, it kind of felt like I was getting up on stage and waving to 5-year-olds."
Ooft. That won't sit well with Swifties.
Charli XCX is done with being an opening act for any artist now, which she reiterates in the coverstory.
"I've done so much of it, and it really cemented my status as this underdog character, which I like now," she said to Pitchfork. "But I need to just own my own f--king s--t finally."
Shortly after Charli XCX's interview went live, she took to social media to clarify her somewhat controversial comments.
"Hey [heart emojis] a few people on the internet have taken something I said out of context and I want to clarify there is absolutely no shade and only love here!" Charli XCX wrote. "As I say in the article and have said many times before, I am extremely grateful to Taylor for inviting me to open for her. She's one of the biggest artists of my generation and the reputation tour was one of the biggest tours in history.
"In the printed version of this much wider conversation my answers about this tour were boiled down into one kind of weird sentence," Charli continued.
"Leading up to that tour I'd been playing a tonne of 18+ club shows and so to be on stage in front of all ages was new to me and made me approach my performances with a whole new kind of energy – more so I talked about how it was brilliant opening for Taylor, I am extremely grateful for the opportunity I was given and how much fun it was to perform to a new audience!"
"Any tour I've been on with any other artist as an opening act has always taught me so much," she added. "It's always wonderful to watch other artists do their thing, own their stage and speak their language to their dedicated fan bases – and this was especially true for the reputation tour."
When Charli XCX and Camila Cabello ended their tour with Taylor Swift in November 2018, she wrote how "happy" she was that she got to be part of such a "huge tour" with "great people".
"OMG TOUR IS OVER!!!! IF U SAW ME LIVE THIS YEAR COMMENT THE NAME OF THE CITY U SAW ME PERFORM IN BELOW!!! I wanna reminisce!!!!!" Charli wrote to her fans on Instagram.
"I feel happy and sad all at once. Happy that I got to be a part of such a huge tour with so many great people (example - @taylorswift @camila_cabello @broodsmusic), happy that I got to perform my songs in such huge rooms to so many new people, happy that a lot of those people actually knew those songs (LOL!) and happy that I got to throw my own parties along the way for the hardcore angels who like to rave until 5am once everyone else has gone to bed hehe. Sad because... well it's all over now!!!"
We don't think Charli meant any harm in her comments to Pitchfork, but let's be real - opening for Taylor Swift is something added to her resume that she should be EXTREMELY proud of!