It was a dream come true for No Doubt lead singer Gwen Stefani that even when her voice was not working that day, she had to come to the studio to collaborate with master rapper Eminem on Kings Never Die, one of several new Eminem tracks featured on the upcoming boxing drama Southpaw, starring Jake Gyllenhaal.

Gwen, 45, confessed that she had been dreaming of working with Eminem to collaborate on a song, reported TV3.

"He's so incredible. He's a real artist. He's a real weirdo," the singer told Entertainment Tonight. "I just finally got the call. It was one of those days where they were like, 'Can you come do this?' And I was like, 'Really? Really?' And they were like, 'It has to be done today.' And I was like, 'Ok!' And I just made it happen.

"I went down there and my voice happened to not be working that day, and I was like, 'Ok perfect -- like, the one time I get asked to be on the Eminem song!?' I don't know - I just prayed really hard and the engineer made it come to life," Stefani narrated.

Her voice almost prevented her from finally realizing her chance to work with Eminem, whom he admires because of his unique personality. But when she went to the studio, everything went fine.

She told Entertainment Weekly, "The process was really simple. One of his guys came and played me the song and then I was like, 'Yeah, I like it! Amazing!' "I've always wanted to work with him - he's a genius - and then all of a sudden they called me and said, 'It has to be done today', and I was like, 'Ah, my voice' - I had a cough or something. I went in and I made it happen, but it was like, barely."

Stefani said she was impressed with the outcome of her collaboration with Eminem,"It was like, 'God, why does today have to be the day my voice is not happening?' But it ended up turning out good. I wanted to do something with him for so long and I feel honoured. He's very selective. I feel really honoured."

But the craziest part, Gwen said, was when she received a phone call from Eminem to thank her for doing the song with him.

"I'm driving, literally driving in the minivan with all my kids in the car... and on Bluetooth, somebody calls. And I'm like, 'Hello?' And it was him!" she continued. "And he's like, 'Hey just wanted to say thanks for doing the song. And I was like, 'You don't understand- I have all my kids in the car. You can't call me right now. All my kids are in the car! Don't say any bad words!' I was literally going to soccer practice. But I was like, 'I'm gonna make your song so cool!'"

The movie Southpaw, which also stars Forest Whitaker and Rachel McAdams, will hit the U.S. theatres on July 24.