'She's kinda like a unicorn!' Sia surprises contestants by revealing her face on American Idol as show reaches final four

Sia has famously found fame despite always hiding her face.

But the pop superstar opened up completely on Thursday as she coached the final four in American Idol to sing her own songs - crying during rehearsals and even letting them see her face.

While viewers only saw her behind her trademark black-and-white ‘curtains’ wig, with a large blue bow in her hair, she opened up for eye contact during her mentoring sessions, with the camera mostly shooting her from behind or neck down but occasionally pixelating her face.

Curtain wig: Sia hid her face from viewers on American Idol on Thursday but not from the finalists

‘To see her in the flesh is kinda like seeing a unicorn,’ admitted Trent Harmon, while La’Porsha Renae raved about her blue eyes and saying of seeing her face: ‘It’s beautiful.’

Sia, in a short blue dress and high heels, mentored the finalists as they got down to the wire, with Sonika Vaid, 20, the latest to be eliminated.

The remaining four finalists raved about the advice she gave them, helping spark two performances that left judge Jennifer Lopez watering up with tears of emotion.

New song: The Australian singer also performed her new single Cheap Thrills

Blue eyes: La'Porsha Renae revealed that Sia has blue eyes

Sent home: Sonika Vaid was eliminated from the singing competition show

But more surprisingly, Sia had tears of her own during rehearsals, welling up as La’Porsha, 22, rehearsed Elastic Heart, telling the hopeful: ‘You’re gonna make me cry!’

‘I stood before these kids and they were singing songs that I had written - and I kinda wanted to cry,’ Sia admitted.

‘It was much more meaningful than I had anticipated. It was really quite moving,' she said.

Wanted to cry: Sia got emotional listening to the contestants perform song she had written

Encouraging them all to ‘become a part of my weird gang,’ Sia - who also performed herself, singing Cheap Thrills with three dancers - soon had them posing in her trademark wigs.

And she also dished out her own quirky brand of advice. 

She told La’Porsha to ‘stay true to who she is,’ and later added: ‘And if anyone were to tell her to do something different I feel like I would want to high karate kick them in the throat.’

She added with a smile: ‘It’s just the kind of gal I am.’

Quirky advice: The singer warned anyone who wanted to change La'Porsha

But not all of her advice was well received - with judge Harry Connick Jr failing to hide his anger at her telling MacKenzie Bourg, 23, to challenge himself and hit a high note he was convinced he could not make.

Harry insisted the advice ‘really irritates me,’ claiming it turned his performance of Sia’s Titanium into ‘an ice skating competition, when everybody’s waiting for the triple axel.’

‘It’s not about a note, it’s about the song,’ Harry said, clearly frustrated.

Clearly frustrated: Harry Connick Jr was clearly frustated that MacKenzie went for a crowd-pleasing high note

‘I’m not a fan of that. Hit the note, don’t hit the note, it doesn’t matter. I’m not a fan of that. You’re smarter than that,' he proclaimed.

But the other judges all backed up Sia and MacKenzie, who had proudly said it was the first time he was ‘being really pushed vocally’ by anyone.

‘It was great that Sia made you go for that,’ J-Lo insisted. ‘I don’t think life begins until you’re out of your comfort zone. She’s right - you can do it, you should have done it, and you did it.’

Comfort zone: Jennifer Lopez however applauded MacKenzie for leaving his comfort zone

While La’Porsha had made Sia cry in rehearsals, it was Trent Harmon, 25, who first made J-Lo well-up with his powerful performance of Chandelier, again encouraged by the star to hit some incredible high notes he would not normally have even considered trying.

‘From the first note to the last note I gave it 100 out of 100,’ Harry told him

Keith Urban meanwhile called it ‘beautiful’ and insisted: ‘It was exquisite I loved every minute of it.’

Powerful performance: Trent Harmon worked out Chandelier with Sia during rehearsal

Getting emotional: J-Log got emotional as Trent performed

Saying he had family members who struggled with alcoholism, Trent admitted he was thinking about himself while performing, telling the judges: 'I’m kinda swinging from the chandelier, y’know. I’m doing what i love to do - I just did it.’

‘You’re a contender baby,’ J-Lo told him proudly.

But she seemed even more emotional when Dalton Rapattoni - who bonded with Sia when she told him she was bipolar just like him - broke down sobbing after a raw and emotional performance of Bird Set Free.

Kindred spirits: Dalton Rapattoni learned that Sia also had been diagnosed with bipolar disorder

Hugged it out: Sia and Dalton hugged it out after rehearsal

Telling him that ‘everyone is cheering for your bravery,’ J-Lo told the 19-year-old: ‘You are such an artist, and it’s beautiful to watch you. You touch all our hearts, especially tonight. It was just really beautiful to watch that happen.’

Harry told him it was ‘something you needed to do,’ getting loud cheers as he said: ‘I don’t know how it sounded at home but I sure liked it in here.’

Keith, meanwhile, told him: ‘I’ll take an inspired attempt over soulless perfection any day of the week. It was fantastic.’

Nailed it: Dalton impressed the audience and judges with his performance

High praise: Jennifer praised Dalton for his raw performance

Inspired attempt: Keith Urban also praised Dalton for his inspired performance

As well as Sia, the top four also had to perform a rock classic, this time coached by E Street Band legend Steven Van Zandt as well as the show’s mentor, Scott Borchetta.

Trent again stunned the judges - and again thanks to some inspired advice from the guests, with Little Stevie encouraging him to make ZZ Top’s Sharp Dressed Man ‘swampy, swampy, swampy!’

After a slow, soulful opening that completely rearranged the song, Trent and the band kicked into the classic riff halfway through - still keeping the soulful feel.

Sharply dressed: Trent Harmon killed his version of ZZ Top's Sharp Dressed Man

‘That was so freakin’ good!’ Keith told him. ‘Like, ten out of ten, man. Fantastic.’

‘I’ve been waiting for you you to have this performance,’ J-Lo told him, calling it ‘so good’ and ‘so sexy.’

‘I knew it was in you - it was perfect,’ she insisted, while Harry got so excited he ran out and picked up host Ryan Seacrest as they joked about a Spanish word J-Lo used to refer to it being dirty.

While the mentors helped perfect Trent’s performance, Dalton initially clashed with them when they bluntly admitted that his planned song, The White Stripes’ Seven Nation Army, wasn’t working for them.

Special guests: Steven Van Zandt and Scott Borchetta advised the finalists on their rock songs

NOt working: Dalton stuck to his guns before switching to his back-up track God Only Knows

After initially digging in his heels, he finally realized during rehearsals with the full band that they were right - and switched last minute to his back-up track, The Beach Boys complex classic God Only Knows.

His slow, piano-led version saw him engrossed in the song, sitting down at one point to sing directly to members of the audience.

Calling it ‘really beautiful,’ J-Lo insisted: ‘You really won with that moment, it was a really good moment for you.’

Good moment: Jennifer told Dalton that he won the moment with God Only Knows

As he struggled to make his song choice, he had said that he ‘only gets a minute and 30 seconds every week to talk to America, and I want to pick the right words’ - words that Keith praised as showing an ‘extraordinary attitude.’

‘And to see you sitting on the step here and watching these girls reactions while you’re singing, that’s as pure as it gets,’ the country star told him proudly.

La’Porsha Renae, 22, had been up first, and showed by far the most dedication - admitting it took an incredible 15 hours to prepare her ‘new do.’

New do: La'Porsha admitted it took 15 hour to prepare her new hairstyle

She also had one of the toughest challenges to get into character, singing Bon Jovi’s classic cowboy ballad Wanted Dead Or Alive, with Scott admitting during rehearsals he was ‘concerned’ that the song ‘doesn’t have anything to do with you.’

‘You see her singing, “I’m a cowboy?”’ he asked Little Stevie.

But the E Street Band legend said it ‘was working’ for him, praising the ‘unexpected’ and said she ‘gets away with it’ because ‘she’s just so cool, she’s just so good.’

The judges clearly agreed, with J-Lo telling her: ‘Your voice is so undeniable you could sing the phonebook and it wouldn’t matter.’

Undeniable voice: The singer took on Wanted Dead Or Alive without any problem

Keith praised her for bringing out the ‘blues undertones and the soul and the swagger’ of the song, while Harry, saying it reminded him of Tina Turner, adding: ‘You’re ability as a singer transcends every genre.’

After her Sia song, J-Lo called her ‘a fully realized artist’ who she just watched and did not judge, and Keith added: ‘I feel like I’m at an awards show watching you come out and sing your nominated song.’

MacKenzie’s rock classic - a slow, acoustic take on Cheap Trick’s pop rock classic I Want You To Want Me - had by far the most scathing reception from the judges.

Bad idea: MacKenzie slowed the Cheap Trick song I Want You To Want Me down too much

‘I’m just gonna be straight - I hated the arrangement, I really did,’ Keith admitted, saying he gave it a ‘maudlin energy’ despite being a ‘killer singer.’

J-Lo agreed, telling him: ‘You MacKenzied it - I don’t know if I love how you Mackenzie it. For me it wasn’t the best.’

As well as Sia singing, season five runner-up, Katharine McPhee reprised her iconic performance of Over The Rainbow - with a member of the crew who is a huge fan coming on to hug her.

Classic song: Katharine McPhee and sang her signature song from the show Over The Rainbow

Huge fan: Ryan Seacrest brought a backstage crew member up front to meet Katharine

Season seven winner David Cook also performed his new song Heartbeat with iconic scenes from previous Idol seasons on a screen behind him.

Afterwards, he spoke proudly of how he ‘got married last summer’ and will be best man this summer as his brother's wedding.

Ryan joined him and said he was due to be ‘man of honor’ at his sister’s wedding and David joked: ‘You’re going to look good in a dress, man.’

Good one: David Cook cracked a wedding joke at Ryan's expense