We adapt quickly. It’s part of the human spirit, whether we are teenage tennis stars or the people who line up and take a seat to watch them in the world’s biggest tennis stadium.

Two weeks ago, the vast majority of us had never heard of Leylah Fernandez or Emma Raducanu. Fernandez had never been past the third round in a major tournament and had struggled to find her best form in recent weeks. Raducanu joined the tour in earnest only this summer and had to make it through the off-Broadway qualifying tournament to secure a spot in the U.S. Open.

But by Saturday, when Fernandez, 19, and Raducanu, 18, took to the court for one of the most unlikely Grand Slam finals, we already had a connection.

They had boldly worked through the women’s draw during this special U.S. Open, which was full of communion between the players and the public after all the distancing of the last year and a half.

By Saturday, those who had been following their unexpected progress already knew about their strengths, their multicultural backgrounds and even their quirks: Fernandez’s jig behind the baseline before walking forward to serve, Raducanu’s habit of blowing on her fingers between points as if to cool off a very hot hand.

But what was most striking on Saturday was how quickly both unseeded players adjusted to this grand occasion, calmly giving thoughtful prematch television interviews, walking past Billie Jean King’s quote on the tunnel wall, which says that “pressure is a privilege,” and then walking past King herself as they emerged into the late-afternoon sunshine for the biggest opportunity of their short careers.

It was all new, but you would not have known it once the ball was in play, as both attacked their groundstrokes and did their best to seize the occasion even after having nearly two full days to think about the showdown once they had won their semifinals.

After the introductions, Fernandez ripped a backhand crosscourt winner on the opening point. Raducanu later pounded a backhand winner of her own to hold serve and win the opening game.

Grand Slam finals, even with more experienced players, can too quickly become one-way traffic. Tennis is a game of momentum, and the best-of-three-set format used by the women allows less time to turn the tide than the best-of-five format used by the men.

But Raducanu and Fernandez both held firm, extending rallies with their quickness and defensive skills on the move, but more impressively finishing points with authority when they had created the space to go for winners.

Their styles contrast in some ways. The left-handed Fernandez uses more spin and enjoys deploying the drop shot. The right-handed Raducanu favors more direct power and has an ability to run around her backhand in a flash and rip an inside-out forehand that Roger Federer could relate to.

But Fernandez and Raducanu are very contemporary tennis talents in their ability to sustain pace and consistency from low body positions, their knees often touching the court.

Some of their extended rallies were spectacular on Saturday as they exchanged two-handed backhand bolts with nary a grunt, their sneakers squeaking on the hardcourt as they focused on becoming a U.S. Open champion.

Only Raducanu would get that satisfaction, and though the score of 6-4, 6-3 will look fairly lopsided in the history books, anyone who watched them will know that the match was much more tenuous than that.

“These two young women are a gift to tennis, an absolute gift,” Andy Roddick, the 2003 U.S. Open men’s champion, wrote in a post on Twitter.

Fernandez is not yet a Grand Slam champion herself, but she is a world-class fighter who walks between points with the steely determination of someone on her way to break up a bar brawl.

And after losing a back-and-forth first set, she had every reason to still believe in her chances after all the times she had battled past top players at Flushing Meadows. She had upset three players ranked in the top five — Naomi Osaka, Elina Svitolina and Aryna Sabalenka — as well as Angelique Kerber, a former No. 1 in resurgent form.

Fernandez had beaten them, so when Raducanu took a 5-2 lead in the second set but was unable to convert her first two match points on Fernandez’s serve, Fernandez grinned between points as if she knew something that nobody else yet suspected.

Why should she not have believed in another comeback at this late stage? But when she got a break point in the next game, she had to wait to play it as Raducanu, who had scraped her left shin while sliding for a shot, took an injury timeout to clean up trickling blood and have the injury bandaged.

It was within the rules, but in this thinker’s sport of ebbs and flows, it was a stoppage that perhaps made the difference. Fernandez, after expressing her displeasure with the pause to officials, pushed a forehand long, and Raducanu then saved a second break point with a leaping tap of an overhead.

She was back to deuce with Arthur Ashe Stadium abuzz and presumably most of Britain wide awake, as the match was broadcast in prime time in Raducanu’s home country.

This time, she did not flinch, surprising Fernandez with a fine serve down the T that gave her command of the rally and brought her a third match point.

She mulled her options, tossed the ball high and smacked an ace to become the first qualifier in the long history of tennis to win a Grand Slam singles title. In 10 matches, she never lost a set.

“I never thought I would see it, so I’m in shock,” said King, who watched from the stands as Raducanu dropped her racket and fell to the court, her hands covering her face.

It was a transformative moment, one that left both players in tears. But what seemed remarkable when the match ended was the same thing that had seemed remarkable as it began: the poise and adaptability of both young finalists.

And as Fernandez took the microphone, her eyes still red, she had the presence of mind to say what she had planned to say on this bittersweet Saturday, the 20th anniversary of the Sept. 11 attacks.

“I know on this day it was especially hard for New York and everyone around us,” she said. “I just hope I can be as strong and resilient as New York has been the past 20 years.”

David Waldstein contributed reporting.