Told by Simon Cowell that her voice was “almost annoying” when she auditioned for American Idol, Tori Kelly did the sensible thing and took no notice. Five years later, having self-released two EPs and had millions of views of her YouTube channel, the 22-year-old Californian has just reached No 2 in the US with her debut album, Unbreakable Smile. At the MTV VMAs last Sunday, her scorched-earth a cappella treatment of current single Should’ve Been Us was one of the most blogged-about moments of the night.

At this solo acoustic show, Kelly is an agreeable mix of fresh talent and aw-shucks modesty. On the one hand, she’s the most vocally over-endowed pop singer since Mariah Carey – and almost reaches Carey’s whistle register on the opening Where I Belong – but utterly lacks her diva genetics. If anything, she steers closer to Carly Rae Jepsen’s perky cheerleader mannerisms, but blame it on the fact that she’s still getting her head around the sudden career-surge and doesn’t quite know how to deal with fans who harmonise with her on album tracks that haven’t yet been released here. When she says, mid-show, “I’m not going to forget where I came from,” you’re inclined to take her at her word.

One of the points of difference between her and other pop stars, such as her physical lookalike Rita Ora, is that she plays guitar. Rather than indicating an indie bent, it’s there as a means of grounding herself by reflexively strumming. She bashes it up at times, though – on Frank Ocean’s Thinkin Bout You, she’s nearly shredding. Added to her tornado of a voice, it makes for a dizzying moment. With more than a few songs in tonight’s set kicking back at a manipulative music business (on Unbreakable Smile, her voice elegantly curdles as she sings: “You could dress up some story, saying ‘Tori, this’ll sell more records’”), Kelly could be a maverick in pop star’s clothing.

• At the Tabernacle, London, on 28 September (sold out) and Koko, London, 16 October.