Gerwyn Price is calm now. On a grey Thursday in the Welsh valleys, he is thoughtful and reflective as he ponders his unlikely journey to the top of a sport he barely played until adulthood. Six years ago, he was a professional rugby player for Neath, throwing a few arrows in his spare evenings. This week, however, he will step out at Alexandra Palace in London as the coming force of darts: the two-time Grand Slam champion, the provisional world No 2 and one of the most electrifying and controversial characters in the sport.

How it all happened is the question everyone wants to ask. He does not quite know the answer, but what he does know is that when he pulls that shiny shirt over his tightly coiled frame, it may as well be a superhero’s cape, awakening something in him. The adrenalin begins to course. He is prone to bellow in opponents’ faces at full volume. And he starts playing devastating darts. In a game dominated by psyche, Price has created a feared character with no apparent ceiling.

“A lot of people can throw darts,” explains Price. “It’s just how they think about the game. That’s how I’ve improved. My mind is different.”

As recently as 18 months ago, the 34-year-old Price was still essentially a journeyman player with a few notable wins to his name. That all changed at last year’s Grand Slam of Darts, when he beat double world champion Gary Anderson in a heated final – more on that in a sec – to win his first major TV title.

Even more incredibly, he defended it last month, brushing aside the formidable world No 1, Michael van Gerwen, in the process. The Dutchman, still overwhelmingly the man to beat, firmly denies the upstart challenger has got into his head. Which is probably an admission that he has, a little bit.

For Price, who turned professional in 2014, it comes down to ambition. “Eighteen months ago,” he says, “you turn up and you’re like: ‘Just get through the first round.’ I wasn’t bothered about winning tournaments at the beginning. I just wanted to win enough money so I could stay on the tour, earn a living for my family. But then when you start to win, you just want more and more.”

Not everyone has been thrilled by Price’s rise. That first Grand Slam title was marred by a spat with Anderson, taking umbrage at his aggressive, extravagant celebrations. Price’s first major trophy was awarded in a hailstorm of boos from the Wolverhampton crowd, his antics earning him the biggest fine issued in darts – £21,500, later halved on appeal. His family have been targeted by social-media abuse. Fellow rising star Michael Smith has warned Price that he needs to “learn when to shut up”. Put it this way: in every walk of life, there are certain people who just seem calibrated to put noses out of joint. In darts, Price seems to be one of them.

Very little of this abrasive stage persona survives the transition back to the real world. As we talk at his local golf club near Caerphilly, he reckons now that the whole bad boy of darts thing got a little out of hand. “I’m not the only person that shows emotion,” he says. “The majority do and it’s hard to hold back. People think I do it to put people off. I just need it to get myself going. It didn’t really bother people when they were beating me.”

The great Sid Waddell once compared darts to a Christmas cracker: you need the wrapping on the outside just as much as the bang in the middle and for a sport still trying to fill its Phil Taylor-shaped void and desperately in search of new heroes and antiheroes, perhaps Price was just a handy heel. “You were turning up to games and one or two people would boo,” he remembers. “Then a few more would join in. I was painted as the villain of darts. I accepted it to a certain extent. But it got a bit over the top.”

The low point came at last year’s world championship, when Price was relentlessly barracked during his second-round defeat to Nathan Aspinall, his missed doubles cheered. Did it affect him? “Obviously you notice,” he says. “You don’t mind booing through games. The crowd pay their money, and that’s how you earn a living. I can take a bit of the pantomime villain. But if you’re going for a double, that’s when you should just get a little bit of respect.”

The irony is that taken in its entirety, Price’s story is a rather uplifting tale: a testament to vision and opportunity, the courage to jack in one dream and pursue another. Having left school at 16 with aspirations of playing international rugby, Price represented Wales at all the age-group levels without quite fulfilling his potential at senior level. Aged 30, and with injuries piling up, he began to plot his next move. That was when a side door unexpectedly opened to him.

Price hadn’t thrown a dart until leaving school. There was a board in the bar of the local rugby club where he worked and occasionally if it was a quiet morning Price would have a little throw. He wasn’t particularly good, but later he would start playing in the pub on Friday nights, and discovered he had a natural aptitude for the game.

He won a cluster of local tournaments against grizzled veterans, went down to the PDC’s qualifying school on a whim and to everyone’s surprise, not least his own, scythed through the draw and won a tour card.

Price hung up his boots, kissed goodbye to his modest Neath salary of around £25,000, and threw himself into the arrows. He quickly found his first sporting career a valuable asset. Not only had it taught him the knack of raising his game under pressure, as well as the dangers of over-practice, but in a world of pot bellies and poor diets, Price’s front-row physique actually provided him with something of an edge.

“On Pro Tours [smaller weekend tournaments], it doesn’t really matter,” he says. “But when it comes to the big TV events, the longer formats, that’s when it matters. It’s hard to be carrying a lot of weight in the heat. You’re going to be sweaty, your hands are going to be sweaty, you’re not throwing the same darts because you haven’t got the same grip. If you’re overweight, those factors can put you off your game.”

With the world championship comprising a three-week slog in north London over the festive period, only the strongest will survive. Price has never gone beyond the last 16 at the Palace, but such has been his hot streak that bookmakers make it virtually a two-horse race between him and the redoubtable Van Gerwen, going for his fourth world crown. “He’s the best player in the world, but he’s there to be knocked off,” Price insists. “On his day, he’s practically unbeatable. But on my day, so am I.”

There are times, he admits, when the ex-rugby player line rankles with him slightly. “This rugby thing has got to be parked now,” he says. “I’ve been playing darts for six or seven years. I’m a professional. That’s how I’d rather be known.”

If he ends up lifting the world championship trophy on 1 January, you suspect he’ll get his wish.