Katy Perry had to force herself onstage in Brazil for the biggest concert of her life - because she was emotionally and physically drained after months of jet-setting.

The Firework singer admits she almost walked away from the Rio de Janeiro show last year (11) because she didn't think she could perform at her best - and her new concert movie Part of Me features footage of her battling emotions and exhaustion in the days before the gig.

She says, "Right before Brazil was a very difficult moment. I was, like, so exhausted. I'd been flying around the world.

"You see... me not knowing whether I should go onstage, but it's, like, the biggest show of my whole tour; it was, like, over 20,000 people, so I couldn't let them down - and all their babysitters."