When it comes to credentials for a long career in pop, winning The X Factor has proved about as reliable as a degree from Trump University. However, there has been one unexpected beneficiary of the endless stream of starry-eyed warblers – the world of stage musicals, which have long provided a safe harbour for former contestants.

In the UK, Pop Idol winner Will Young went on to star in Cabaret, while X Factor semi-finalist Diana Vickers soon found herself treading the boards in The Rise and Fall of Little Voice in London’s West End. In the US meanwhile, American Idol has sent a busy succession of contestants to Broadway, from Clay Aiken, who had a run in Spamalot, to 2007 winner Jordin Sparks, who performed in Lin-Manuel Miranda’s pre-Hamilton hit In the Heights.

Now, however, there’s been a transatlantic exchange, with the most successful ever winner of The X Factor UK landing directly on the Great White Way. On Sunday, the musical Cats officially opens, with Leona Lewis donning whiskers and a tattered tail to play Grizabella, the Glamour Cat. The show revives the Andrew Lloyd Webber juggernaut, whose lyrics are based on TS Eliot’s Old Possum’s Book of Practical Cats. (Its most famous song, Memory, also bastardizes the poet’s Preludes and Rhapsody on a Windy Night.)

Cats’ original production started in 1981 and ran for 18 years (21 in the West End); this upgraded version, with a young cast performing their paws off, also looks poised for success. Lewis took the role after Nicole Scherzinger pulled out at short notice. With a strange circularity, the former Pussycat Doll, who had performed the role in the 2014 West End revival, had decided to take up an offer to become a judge on The X Factor instead.

“It’s so weird,” muses Lewis about the coincidence as she sits on a sofa in her dressing room on a sweltering midweek afternoon, wearing the regulation 2016 summer uniform of a giant denim shirt dress. “I guess it’s a small world, really.”

Leona Lewis and the cast pose outside the theatre. Photograph: Bruce Glikas/FilmMagic

She was asked to join the cast by Lloyd Webber, who told the Economist that he was “furious” when Scherzinger quit. Lloyd Webber called Lewis to ask whether she was interested, but insisted that she audition. Being a seasoned trouper, she flew to New York (she has homes in Los Angeles and London) and within a few days the part was hers. She will play Grizabella until October, not a long stint by theatre standards. “I’d just come off a tour, I was having some time off and it just seemed like the perfect role because it’s not a long commitment.”

Until One Direction came along, Lewis was the most successful X Factor contestant ever. Poised and humble, with a stratospheric voice, she won the contest in 2006. A year later her second single Bleeding Love was No 1 in 35 countries, including the US, while its parent album Spirit sold 9m copies worldwide – not quite as much as the 15m American Idol winner Kelly Clarkson achieved with her second album, Breakaway, but still serious numbers.

After a few years, however, she and Simon Cowell, the X Factor judge and her label boss, came to disagree about which musical direction she should go in (she made a dubstep-tinged album inspired by Kate Bush and Tears for Fears; he wanted her to do Motown-style covers) and Lewis left his label. A subsequent stint with Island Records ended a few weeks ago, and now Lewis is moving on. This week she has been recording We Are Free from the Gladiator soundtrack with legendary film composer Hans Zimmer; she is also working on a single to come out later this year. Two years ago she alarmed some fans by writing an open letter in which she described the stress she had been under as her relationship with Cowell’s label deteriorated, but now Lewis says she feels creatively unshackled: “Having had a decade from doing [The X Factor] to now, I’m freed up to do things that I have always wanted to do.”

If Lewis picked Cats as to remind the public of her indisputable talents, she chose well. She gets the big number, Memory, coolly drains it of most of its schmaltz, and then belts it with such passion and precision it probably ends up in Times Square. The pressure of handling such a showstopper doesn’t faze her: Lewis is a stage school kid who first saw Cats aged seven. “I know the song, I’ve sung it since I was a little girl – it’s been exciting [to perform], more than too much,” she says.

She’s used to high-stakes performances. Two years after The X Factor, in which she had to crush the competition in front of an audience of millions every week, she performed Whole Lotta Love with Jimmy Page at the handover ceremony at the Olympics in Beijing. Page described it, quite sincerely, as his proudest moment outside Led Zeppelin. “We spent loads of time together in Beijing and he’s like a lovely uncle, so kind,” says Lewis of the once notorious guitarist. “All the iconic rock stars I’ve gotten to meet are like that. They are all so chill, and have no ego – they’re very confident and secure in themselves.”

Lewis is a vegan and animal rights activist. She refused to open the Harrods sale because they sell fur coats, said she would give up her career in return for a global ban on vivisection, and even managed to stop her German boyfriend, brought up on bratwurst, from eating meat. Did her empathy for the animal kingdom influence her interpretation of the role – a moth-eaten kitty shunned by her fellow felines, but taking comfort from memories of her glory days?

Lewis winning the X Factor in 2006. Photograph: Ken McKay/REX

She’s gracious enough to take the question seriously. “It’s weird, isn’t it, because I also got offered a part in The Lion King when I was younger,” she says. “There is definitely something for putting that kind of vibe out there – maybe that played a part in me taking on the role of an animal.” As for real-life felines, she feeds a stray cat that stalks her land in LA, but is more of a dog and horse person. “Cats are more standoffish. But it depends. My cousin’s got this big tabby cat that I’m in love with.”

Lewis is not the kind of pop star who mouths off, gets into Twitter feuds or makes controversial videos. In the age of Kanye and Rihanna, where outrage is a potent currency, that can make her seem an anomaly. Yet along with animal rights, there are other issues that she feels moved to speak out about – most recently, the massacre of 49 people at Pulse, the gay club in Orlando, Florida. “It struck a chord,” she says. “I think of my friends that could have been there, or it could have been me that was there. Putting yourself in that position is just so devastating. It’s terrible, this world we’re living in now and it just seems to get worse and worse. Every day you’re turning on the news and some other tragedy has happened. It’s awful.”

So what’s the role of a performer in all this – to provide escapism from the horrors of daily life? Lewis disagrees. “When people come and see a show I hope that they take away something from it,” she says. “The show’s got a lot of heart. Grizabella’s someone that’s been outcast and the storyline is about acceptance and love. That is a deeper reason as to why I do music. I don’t think it’s escapism at all, actually. I think it’s really meaningful. It’s about people coming away feeling moved and having a bit more love in their heart.”