Alyssa Raghu decided to make a last-ditch effort to stay on the show after learning she didn't have enough votes to automatically make it to the Top 10.
The 17-year-old from Orlando, Fla., donned a sparkling, sleeveless jumpsuit as she took the mic and sang a ballad-like twist on Perry's fast-paced 2010 hit.
At the beginning of the performance, cameras panned to a grinning Perry. As Raghu held her final note, all three judges stood up to applaud the teary-eyed teen before giving her feedback.
"First of all, how dare you!" joked Perry, as the 17-year-old wiped away tears from her eyes. "How dare you come out and sing that song 100 better than me."
"No, I could never. I could never," repeated Raghu as Perry praised the "beautiful" rendition.
The 34-year-old artist then encouraged Raghu – who was cut from the competition the season prior – to keep perfecting her craft, echoing advice Luke Bryan's gave earlier.
"I think this has been a really important master class. It's like the second semester. And I think you've learned so much and you've got so much time, and I just think that it's going to happen if you ... keep applying yourself the way you keep applying," continued Perry.
Lionel Richie credited Raghu's "heart and drive" for keeping her going, despite rejection in the past.
"The sky is the limit for you. We are in love with you, and I hope America will see your face very soon. Do not think that this is at all goodbye," concluded Richie.
It turns out Richie was right. At the end of the show, Perry saved Raghu and secured her a seat in the Top 10.
"And @katyperry still believes in the SAUCE! @AlyssaRaghu is in the Top 10! #AmericanIdol," the show confirmed in a tweet, posting a GIF of a thrilled Raghu.
Perry admitted it wasn't an easy feat.
"I don't know if anyone noticed but I almost broke down in tears having to pick one of my children #americanidol," the singer tweeted Monday night.
But many fans praised Perry's choice.
"YAY!!!! LOVE HER! #AMERICANIDOL," one Twitter user replied.
"Thank you for picking Alyssa! She deserves this!" another exclaimed.
"YESSS MY GIRL MADE IT," another wrote, along with a crying emoji and hearts.