'American Idol': Katharine McPhee Joins Fan Favorite Willie Spence for Powerhouse Duet -- Watch!

The 'Idol' alum returned to the show's stage to deliver a memorable and emotional performance with one of the season's frontrunners.

Sunday's American Idol was jam packed with big performances, memorable moments and celeb guests. However, it was Katharine McPhee's duet with one of the show's frontrunners that stole the spotlight.

Willie Spence grabbed the judges' attention in a big way when he wowed them during his rehearsal, and said his career goal was to win a GRAMMY within the next five years -- an undeniably ambitious goal, but one that all the judges felt was legitimately possible.

Sunday's Idol saw 12 of the Top 24 perform twice each -- one solo performance and one star-studded duet -- which meant there wasn't a whole lot of time to leave a lasting impression.

The fast pace of the episode, however, came to a halt with Spence and McPhee's powerhouse duet, as it demanded attention.

Together, the pair performed a next-level rendition of Andrea Bocelli and Celine Dion’s 1999 classic "The Prayer," and it proved to be the perfect vocal showcase for the Idol hopeful's range.

McPhee -- who was the American Idol runner-up during the show's fifth season in 2006 -- seemed awed by Spence's mind-blowing vocals, and was enamored by his talent and craft.

The song held a special significance for the two as well, as Spence dedicated it to his recently deceased grandfather, while McPhee dedicated her performance to her late father, who died in July 2018.

"You can do no wrong in my eyes," Luke Bryan said after the number. "Part of being a star is making people fall in love with you and, man, I am in love with you, and I am in love with the way you go about being on stage, and the way you go about singing and the place it comes from. It's amazing."

Lionel Richie gave Spence the endearing nickname "Willie Pavarotti" and explained, "In our business, you don't want to be a singer, you want to be a stylist. You have your own lane, and it's just mesmerizing to watch you."

Backstage, an awed McPhee marveled at Spence's ability to take a famous song, and make it his own, and make it something fresh and different and still great.

"He made 'The Prayer,' something that Andrea Bocelli sings, sound like something that Willie would sing," she marveled.

Katy Perry said the duet was "goosebumps central" and that it "really showcased your voice, and different levels... that you have within you."

"I think, to win this, it's about song choice," Perry added. "And it's about those amazing classic songs that other powerhouses sing, and that show that you are also a powerhouse."

On Monday, the next 12 singers out of the Top 24 will get a chance to deliver their two numbers, before eight hopefuls are cut and 18 move forward.

American Idol airs Sundays and Mondays at 8 p.m. ET/PT on ABC.