Andreescu, 19, recovered from a set down to defeat Belgian 25th seed Elise Mertens 3-6 6-2 6-3.

"I'm honestly speechless," Andreescu said. "I need someone to pinch me right now. Is this real life?"

Meanwhile, 22-year-old Bencic put friendship aside to beat Croatia's Donna Vekic 7-6 (7-5) 6-3.

Serena Williams, chasing a record-equalling 24th Grand Slam title, plays Ukraine's Elina Svitolina in the other semi-final in New York on Thursday.

Bencic gets revenge but 'still friends'

Eliminated from the French Open by Vekic in May, 13th seed Bencic got the better of her hitting partner this time round in a match full of baseline rallies.

Vekic, 23, failed to serve out the first set after breaking in the ninth game, with it eventually going to a tie-break which Bencic took charge of.

The Swiss, who knocked out defending champion Naomi Osaka in the previous round, broke twice in the second set, sealing victory on her third match point when Vekic hit a forehand wide.

The pair shared a warm embrace at the net and Bencic was confident their friendship would survive.

"We're both professional enough to be friends off the court and competitive on the court," Bencic said. "I think we did well. I'm sure off the court we are friends still."

Bencic will now return to the top 10 of the world rankings for the first time in more than three years.

She reached a career-high seventh in February 2016 before injuries and wrist surgery left her as low as 328th.

"I dreamed of this like a little kid so now that I'm here I'm really enjoying it," said the Swiss, who ended a five-year wait to improve on her previous best Grand Slam showing of a US Open quarter-final.

Andreescu's dream debut continues

Bianca Andreescu becomes the first teenager to reach the US Open semi-finals since Caroline Wozniacki in 2009

Later on Arthur Ashe Stadium, Mertens made a quick start against promising youngster Andreescu as she raced into a 4-1 lead to move the opening set beyond the reach of the Canadian.

A rapidly improving Andreescu responded with an early break to go 2-0 up in the second set, only to be pegged back by Mertens in her following service game.

That was a temporary set-back, however, as the teenager broke twice more, claiming four successive games to level the match.

With Andreescu threatening to pull away, Mertens saved two crucial break points to draw level at the start of the third set - and was then forced to save two more when trailing 3-2.

The relentless Canadian eventually made a decisive breakthrough with her third break point opportunity of the eighth game, before serving out to complete an impressive turnaround.

Andreescu, who is making her US Open debut, has won 11 matches in a row after winning the title in Toronto in August when Serena Williams retired during the final.

That was her second title of the year after victory in the prestigious Indian Wells event in March.

"I think it's just the experience from playing these huge events this past year," she said. "All that experience is just catching up."