Fan favorite MacKenzie Bourg – who wowed with a version of Whitney Houston's "I Wanna Dance with Somebody" earlier in the season – admitted her songs left him feeling out of his "comfort zone."
But after a mentoring session with Sia, Bourg wowed with a version of "Titanium" and fellow competitor Dalton Rapatoni took "Bird Set Free" up from the key he had originally planned to sing it in. But it was Trent Harmon who received the highest praise from the judges for attempting the high notes on Sia's "Chandelier."
"Sia said, 'Oh! You picked the hardest possible song, good for you,' " Harmon revealed after the feedback. "I wanted to make her proud."
Keith Urban told PEOPLE of the performance afterwards: "Trent attempted one of, to me, the greatest songs out there and he killed it. We all know he can sing but he was pushed to a different level tonight."
Meanwhile Harry Connick Jr. told PEOPLE: "It is really difficult to go out there and do what Trent did tonight. That was a very difficult song and he succeeded technically and, more importantly, he told a full story."
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"Before this competition, I never really thought I could pursue music as a career, but Idol has opened my eyes to the opportunity I have to pursue my passion," she told PEOPLE afterwards. "And I plan on making the most of it."
The Top 4 will perform next Thursday (at 8p.m. ET) and the fifteenth – and final – Idol will be announced the following week on a star-studded finale April 7.