Emma Raducanu returns the ball against Leylah Fernandez on Saturday in New York City. Photo: VCG

British teenager Emma Raducanu completed the greatest underdog title run in tennis history on Saturday, becoming the first qualifier to capture a Grand Slam crown by defeating Canada's Leylah Fernandez in the US Open women's final.

The 18-year-old was the first British woman in 44 years to win a Slam crown after beating 19-year-old left-hander Fernandez 6-4, 6-3 to take the $2.5 million top prize.

"I knew I would have to dig deep," Raducanu said. "It was an incredibly difficult match but I thought the level was really high. I had to play some of my best tennis."

It was a stunning achievement for the unheralded 150th-ranked teen, who didn't drop a set in three qualifying ­matches and seven main-draw matches over two weeks in producing a stunner for the ages.

Raducanu is the ­youngest Grand Slam champion since Maria Sharapova at 17 in 2004 at Wimbledon.

Not since Virginia Wade in 1977 at Wimbledon had a British woman taken a Slam singles title, with Raducanu the first Briton to claim the US Open crown since Wade in 1968.

Raducanu has had the fewest Slam starts of any women's Slam winner, reaching the fourth round in July at Wimbledon in her only prior Slam appearance.

Fernandez, ranked 73rd, had ousted defending champion Naomi Osaka, second-ranked Aryna Sabalenka and fifth seed Elina Svitolina in her epic run to the final.

Raducanu is the youngest US Open champion since 17-year-old Serena Williams in 1999 and the first US Open women's champion not to drop a set since Williams in 2014.

The prodigy showdown was the first all-teen Slam final since Williams beat 18-year-old Martina Hingis for the 1999 US Open title.

"Both of us were playing fearless tennis for two weeks," Raducanu said. "I hope we play each other in many more tournaments and hopefully finals."

After an early exchange of hard-fought breaks, Raducanu broke in the 10th game to take the first set, blasting a forehand winner after 58 minutes.

In the second set, Fernandez broke to 2-1 but Raducanu blasted a backhand cross-court service return winner to equalize 2-2 and later whipped a forehand passing winner by the Canadian to break for 4-2.

Fernandez saved two match points in the eighth game and held to 5-3, then earned a break chance at 30-40 in the ninth.

But on the point, Raducanu skidded on her left knee and opened a bleeding cut. The match was halted while a trainer bandaged the wound as Fernandez spoke with the referee.

"I didn't know how serious her fall was, so that's why I went to see the official," Fernandez said. "It was just too bad it happened in that specific moment with me, with the momentum. But it's sports, it's tennis. Just got to move on."

When play resumed, Fernandez sent a forehand long to squander the break chance and Raducanu won after an hour and 51 minutes on her third ace, leaving Fernandez in tears.