Judges Randall Abrahams said he channelled El DeBarge well while Unathi Msengana jumped up from her chair and Somizi Mhlongo gave a standing ovation.
"It hasn't sunk in yet, but we're getting there," Cox told Sowetan.
Cox, from Eden Park on the East Rand, said he watched You Tube videos of live performances of old R&B musicians on the eve of his memorable Idols showing. "I studied how they maintained their cool and remained calm. I looked at how they used their voices and their facial expressions, and that's what I did on the night."
Cox said he picked Campbell as he's a fan and because he loved the song as well. "I could hit a really high version of that song when I was younger, but now my voice broke and I had to change into a new artist, and find myself again."
He said he had been overwhelmed with compliments since Sunday. "I have no words to express myself." Cox has said that he is in the show in tribute to his mother. "I keep thinking about her in every performance, sitting in the audience and crying. She was proud of me and pushed me to do my best."
Talking about his chances, Cox let slip that he has two men standing in his way.
"I'm very intimidated by Keegan [Martin], please don't tell him, but I'm scared of him. His vocal range is off the charts and Thami [Shobede] has that husky voice I've always wanted. But it's up to SA to decide and I have to keep up the standard if not take it higher."
Cox is the resident performer at the up-market Grillhouse restaurant in Rosebank, Johannesburg, where he performed with Jamali singer Liesl Penniken. He also sang at churches and was part of a group.
"I want to be an inspiration to the youngsters in Eden Park. There are many bad influences like drugs and that's not the only way, there are many other options available to the youth."
He hasn't decided on the next song for Sunday, but said if he emotionally connected with a song and it talked about his story, then it will jump from the page.