The American Idol alum covered the movie's hit song, "Shallow," during her Meaning of Life tour stop in Green Bay, Wisconsin, on Friday, where she couldn't stop gushing about Gaga.
"This chick is amazing. We're really different, but at the same time, we're really similar in the fact that she just loves music, and that's why she's doing what she's doing," Clarkson told the crowd during her "A Minute and a Glass of Wine" segment.
"Honestly, I remember her first single, we actually did a show together, combined for some event... she wrote me handwritten letter saying, 'Thank you so much. It was an honor to open for you.' She was the nicest human being ever," she continued. "It's cool to know someone that talented is also really nice. I've been rooting for her for a while."
Clarkson, who said she's hoping Gaga wins at the Oscars, praised her and Bradley Cooper for doing "such an awesome job" on the song. "Obviously I don't have Bradley Cooper here," she said, explaining her solo version of the duet.
"I hope I don't suck, and if I do, I hope she doesn't see it," Clarkson joked, before wowing the audience with her cover.
Check it out around the 8:30 mark below.
Gaga performed another rendition of "Shallow" at the 2019 GRAMMYS on Sunday, giving it an edgy twist. Backstage, she gushed to ET about how singing the song felt like a "blessing."
"It’s a really special song," Gaga shared. "I know that might sound like not a good thing to say coming from the person that co-wrote it, but we had a special moment in that room together, me and [co-writers] Mark Ronson and Anthony Rossomando and Andrew Wyatt. It was two guitars, a bass, a piano, a typewriter and us talking and singing back and forth and having a conversation."
Gaga said the the song is ultimately a "conversation between a man and a woman and I think that represents a lot."
"All [of us] in this world, we need to keep talkin' and we need to keep talkin' about the things that are important," she added. "Not [about] the shallow s**t, but the deep [things]."