Nicki Minaj thinks it's "great" Britney Spears' upcoming tour will be totally substance-free.

The 'Check It Out' hitmaker - who is opening the star's 'Femme Fatale' world tour - explained with the troubled singer being the "queen of her castle", everyone around her will be more responsible when it comes to avoiding alcohol and other substances.

She told Life & Style: "I think it's a great thing. If that's the way Britney wants it, so be it. She's the queen of her castle. It trickles down and makes everyone responsible.

"We start on June 17, and I'm sure it's going to be a whirlwind. I'm sure it's going to change the way I view touring. Every tour I do is different, and I get new energy."

Nicki - who has appeared on tracks by the likes of Kanye West, Rihanna and Drake and Sean Kingston in the past - recently revealed she is "done" with collaborations, and would only consider performing alongside Britney in the near future.

She said: "I'm done with collabs. No more collabs for the next two years. Although - Britney's a Snatch like me and she's cute. Yes. We might do something."