Ariana welcomed her pal to the stage as she closed out this year's Coachella on Sunday night, April 21, but his lip syncing was a low point for many people who have since slammed the "Baby star for not singing live.
Ariana took to Twitter on Wednesday to back Justin, explaining he had no time to rehearse before taking the stage.
"We also decided to do this ten minutes before my set started," she explained. "we had 0 soundcheck, 0 rehearsal."
She then assured her special guest that she was thrilled to have him, adding, "U were singing with the back tracking like most cameos do. people are bored. people don't know how it feels to be under such scrutiny. the world was happy to have u on stage again."
"Your fans, friends & the world is thrilled to have you back. everyone was so happy to see u smile like that. no one will ever understand how it feels to be u, but it's not their job to. take care. make music. u are loved."
Grande then took aim at Justin's critics, raging, "people are so lost. one day everybody that works at all them blogs will realize how unfulfilled they are and purposeless what they're doing is and hopefully shift their focus elsewhere (sic). that's gonna be a beautiful a** day for them! i can't wait for them to feel lit inside... i don't like when people try to ruin beautiful moments for my friends that's all. have a good day."
Meanwhile, Justin also defended himself against Coachella criticism from Morgan Stewart, the host of E! show "Nightly Pop", who ridiculed his lip syncing guest spot.
"Imagine if you spent even half the time you spend laughing at other peoples expense actually building people up and encouraging people how much positivity you could bring," the pop star tweeted on Wednesday. "What hurts about this is the fact that you have a platform to make a difference. And rather than being positive you belittle people."
"Think about how awesome it felt for me to be on that stage after being away for so long, the excitement and joy it gave me doing the thing i love the most, i sang to backtrack like most cameos do this is a normal thing."
"And rather than try and make people feel accepted and loved you find things to pick apart like the world isn't full of that already.. we can find something negative to say about anything or anyone. When are we gonna be the kind of people that find joy in adding value to one another and not tearing each other apart...."
"I didn't realise it was going to be that bad," Stewart said as she watched footage from Justin's Coachella guest spot during her show on Tuesday night.