Carrie Underwood: `I entered American Idol on a whim`

The 28-year-old won the show in 2005 and has since sold 14 million albums since, making her the most successful star ever produced by the talent series.

The Oklahoma-born country singer drove for seven hours to get to an audition in St Louis, Missouri, but said that she never really expected much to come of it.

She told The Australian, 'I entered American Idol on a whim. I was about to graduate from university. I didn't know what I was going to do with my life, so I thought: 'Why not? The worst they can do is say no.''

The five-times Grammy award winner said that despite her success, she refuses to take anything for granted.

'I don't know if you ever feel completely comfortable in the entertainment business,' she said. 'You never know when something will come along and you realise you still have a lot to learn.'

And she admitted that fame has changed her. 'It would be impossible to not let it affect you,' she said. 'I was 22 when I was on American Idol and now I'm 28. That's a huge growth period in someone's life. I've grown up a lot and learned a lot about myself.'