There’s unexpected and there’s this: a US Open final between two teenagers ranked 73rd and 150th in the world.

That’s where things stand after Thursday’s extraordinary women’s semi-final twin bill at Arthur Ashe Stadium, the latest shocking development at the season-ending grand slam which, despite the absence of some of the sport’s brightest stars, continues to unfold in the most unpredictable of ways.

Thirty-eight months ago, Emma Raducanu and Leylah Annie Fernandez met in the second round of the Wimbledon girls’ singles. Now they will cross paths for the first time since then with all to play for in the world’s biggest tennis stadium: a $2.5m cheque and what surely will go down as the unlikeliest US Open championship on record – regardless of who wins.

“Obviously since then we’ve both come very far in our games and as people,” Raducanu recalled in Friday morning’s dizzying aftermath. “I’m sure it’s going to be extremely different to when we last encountered each other. But we’re both playing good tennis so it will be a good match.”

On Thursday night amid a crackling atmosphere on Ashe, Raducanu became the first qualifier to reach a grand slam final in the Open era with another performance of poise and composure beyond her years in a clinical 6-1, 6-4 win over Greece’s Maria Sakkari. Playing in only her fourth tour-level event, the 18-year-old from Kent has won all 18 sets that she’s played during three matches in qualifying rounds and six in the main draw.

The precocious right-hander, who has dropped a scant 27 games in her six main-draw matches, became the first woman from Great Britain to reach the US Open final since Virginia Wade, who took in Thursday’s matches from the President’s Box. She’s only the fourth British woman in the professional era to reach a major final after Wade, Ann Haydon-Jones and Sue Barker.

“Honestly I just can’t believe it,” Raducanu said. “A shock. Crazy. All of the above.”

It’s been far tougher sledding for Fernandez, who has been pushed the distance throughout a string of three-set upsets over defending champion Naomi Osaka, No 16 Angelique Kerber and No 5 Elina Svitolina ahead of Thursday night’s 7-6 (3), 4-6, 6-4 win over world No 2 Aryna Sabalenka. Fernandez, playing in her seventh major tournament, was already the youngest woman to beat two players ranked in the WTA’s top five at the same major since Williams’ 1999 run. Now make it three.

“I’m glad that whatever I’m doing on court, the fans are loving it – and I’m loving it, too,” said Fernandez, who celebrated her 19th birthday on Monday. “We’ll say it’s magical.”

Their vastly different roads have led to the same destination: the first US Open final between teenagers since 1999, when Serena Williams saw off Martina Hingis for the first of her 23 career major titles.

Now two charmed runs will converge on Saturday in a US Open final unlike any other.

“Here in the US Open I wasn’t really sure how my level was going to be,” Raducanu said. “In a way my tennis level has surprised me in the way that I’ve managed to step up against some of the best players in the world.

“I personally think inside I knew I had some sort of level inside of me that was similar to these girls, but I didn’t know if I was able to maintain it over a set or over two sets. To be able to do it and play the best players in the world and beat them, I honestly can’t believe it.”