'American Idol' Hopefuls Trent Harmon and La'Porsha Renae Command 'Sia' Covers as Top 4 Are Picked

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They were already front-runners to win the final season of American Idol, but Trent Harmon and La’Porsha Renae still seemed to benefit greatly from having Sia as a mentor this week, delivering some of their most powerful and dynamic performances thus far.

The enigmatic singer-songwriter joined the singing reality competition on Thursday as a coach, with each of the final four contestants choosing one of her hits to perform as their final song of the evening. Harmon, Renae, Dalton Rapattoni and MacKenzie Bourg were able to get pointers on their renditions directly from the 40-year-old singer, who was enthusiastic about the opportunity.

"It was very exciting to be asked to come on here and participate," Sia shared. "I was quite excited that I even had a body of work large enough to pick from."

WATCH: Sia Proves She's Every Bit As Weird (and Awesome) As You Imagine on 'Carpool Karaoke'

One of the most emotional performances came from Harmon, who delivered a soulful, original cover of "Chandelier."

"I was such a fan of Sia going into this," Harmon said in a pre-recorded package before his performance. "To see her in the flesh is kind of like seeing a unicorn."

During their rehearsal, Sia encouraged Harmon to fully utilize his vocal range and to try to hit song's the high notes. When Harmon displayed his ability to hit the upper register, it actually gave her goosebumps.

For his live night performance, Harmon -- decked out in a long black coat -- stood in the center of the fog-covered stage to belt out a slower, more delicate rendition of the powerful track, using Sia's advice about hitting the high notes to great effect.

WATCH: La'Porsha Renae and Trent Harmon Bring 'American Idol' Judges to Tears

From the looks on their faces, it was clear that the judges had been moved by his delivery, which was then confirmed with overwhelmingly positive feedback.

"I thought that was phenomenal, I really did," judge Harry Connick, Jr. gushed. "That song tells such a vivid tale, and I really felt that you were completely in another world with that, not to mention how artfully you sang that.”

"From the first note to the last note, I give it 100 out of 100," he added.

Harmon went on to explain that he wanted to make Sia proud.

"I have family members that have struggled with alcoholism, but I tried to make that song more about me," Harmon shared. "I'm kind of swinging from the chandelier, you know? I'm doing what I love to do. I just did it."

WATCH: 'American Idol' Winners: Where Are They Now?

Rapattoni was in the bottom two, but he was saved from elimination this episode when Sonika Vaid was sent home. He made up for his low-end positioning, as his performance of Sia’s “Birds Set Free” ended up being a highlight of the evening.

During their rehearsal, Rapattoni and Sia bonded over their mutual struggle with bi-polar disorder, and that “connection" it made the song "more powerful” for the Idol hopeful. It showed in his delivery, as Rapattoni’s low-key, sorrowful style complemented the lyrics and made the moments where he raised his energy hit even harder.

At the end of his performance, Rapattoni (and his mom!) burst into tears, with the audience bursting into cacophonous applause.

"Everybody is cheering for your bravery," judge Jennifer Lopez explained. "We all want to just let go, we all want to feel good in our own skin, we all want to just have a moment where we just release and we can be ourselves… and singing is obviously that for you."

"You're such an artist, and it's so beautiful to watch you," she concluded. "You touch our hearts."

After the show, Dalton spoke with ET's Sophie Schillaci where her opened up about his tearful outburst after finishing her performance.

"I looked at my mom on the side and my mom was crying, I was like, 'Well you know what, these next few notes are gonna suck because I'm choking back tears for the rest of this song,'" Rapattoni recounted. "But I tried my best to pull it together."

WATCH: 'American Idol': La'Porsha Renae Slays With New Hair and a Beyonce Cover

Rapattoni also revealed what surprised him the most about meeting Sia during their rehearsal session.

"She is so funny and she is so sweet," Rapattoni recounted. "You think, when you listen to her songs, all of her songs are really dark. 'Chandelier,' one of the biggest pop songs of that year, it's about like alcoholism and depression. You think that she's going to be dark and brooding but she's so bubbly and funny. I definitely did not expect that from her."

Fan favorite Renae also continued to bring her tremendous talent to Idol. In the pre-taped rehearsal package, Sia revealed that she was "totally blown away" by Renae's rendition of Rihanna’s “Diamonds,” in February, a song the bewigged singer co-wrote. While rehearsing, the two had a sweet mentor/mentee relationship, with Sia sharing, "My advice to La'Porscha would be to stay true to who she is. If anyone tried to tell her to do something different, I feel like I would high karate kick them in the throat."

Renae did just that in her dynamic performance of Sia’s "Elastic Heart." Backed by two massive kettle drummers, the Idol contestant’s power and attitude was on full display, and the judges were quick to point that out.

"You're such a fully-realized artist, I fell like I'm not even doing my job anymore. I'm not even judging you. I'm kind of just watching you perform and be amazing," Lopez told the 22-year-old singer.

Connick, Jr. agreed, saying, "Have you ever been in a car with somebody that just gets their license, and you're in the passenger seat, and you're terrified? I feel like I'm in the passenger seat of a NASCAR driver when you sing."

"I feel like I'm at an awards show watching you come out and sing your nominated song," judge Keith Urban added.

Renae also opened up to ET about her tearful mentoring session with Sia, who was brought to tears by Renae's vocals during their rehearsal. "It was an amazing experience and for her to cry off of me singing her lyrics, it just really touched me so deeply," she recalled. "I love Sia. I love her, she's amazing."

WATCH: Jennifer Lopez Crushes Hard on 'American Idol' Contestant Dalton Rapattoni After Epic Billy Idol Cover

Bourg, the last of the final four, sang the Sia-written hit "Titanium." While his rendition elicited thunderous applause from the audience, the judges seemed underwhelmed, calling the performance "nice."

Coupled with the judge's outright negative feedback toward Bourg's first performance of the evening, a slowed-down reimagining of Cheap Trick's "I Want You to Want Me," it was a rough night for the 23-year-old musician.

Outside of mentoring the four young hopefuls, Sia also delivered an artistic performance of her newest single, "Cheap Thrills," incorporating all the wigs and bodysuit-clad interpretive dancers we’ve come to expect from the singer.

As the final season of the singing competition show quickly comes to an end, Fox announced on Thursday that Kelly Clarkson, Carrie Underwood and numerous other Idol winners and contestants will be returning for the show's massive, three-night series finale, which kicks off April 5.

WATCH: 'American Idol' Frontrunner La'Porsha Renae Goes 'Full Diva' With Amazing Beatles Cover

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