NEW YORK — Two days ago, Frances Tiafoe was heading into a 50-50 third-round match against Ben Shelton knowing that even if he won, he’d likely be a massive underdog in his next assignment. Defending champion Novak Djokovic was lying in wait in Tiafoe and Shelton’s quarter of the draw.

Sunday night, Tiafoe slung a forehand onto the sideline to reach the U.S. Open quarterfinals, an open field ahead of him and 24,000 people around him under the lights.

From underdog to one of the last men standing, in barely more than 48 hours.

On another electrifying night on Arthur Ashe Stadium, Tiafoe beat Djokovic’s conqueror, Alexei Popyrin, 6-4, 7-6(3), 2-6, 6-3, in just over three hours. He faces Grigor Dimitrov in the last eight, with the wind at his back and belief coursing through him. In tennis, life comes at you fast.

This tournament has so much meaning for Tiafoe, who reached the semifinals two years ago and then the quarterfinals in 2023. The first of those runs was a breakthrough for the American, one which, in his own words, gave him “a s***-ton of confidence.” It sustained him until last year’s event, at which point a bruising defeat to Shelton hit him hard.

“Losing last year to him, I struggled for a long time,” Tiafoe said on Friday, reflecting on a match in which Shelton upstaged him as the crowd favourite, and as America’s favourite new tennis player.

It took until Wimbledon, 10 months later, for Tiafoe to get his mojo back.

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This year’s run feels different. The first was a whirlwind; New York and the American sporting landscape  fell in love with Tiafoe as he showed off his talent. Last year there was more pressure, less joy in Tiafoe’s tennis. Now, there’s pressure again, but it has arrived more suddenly. Two days ago, very few were considering Tiafoe a realistic title challenger.

He is unquestionably in that echelon now, after a largely composed, controlled performance against Popyrin. There were some of the highlight reel moments that Tiafoe lives for, like a whipped forehand passing shot in the second set tiebreak that had the crowd on their feet, but this was also a performance full of margin and smart decisions. Popyrin said afterwards that at times Tiafoe played “scary tennis.”

“If he continues it, he’s got a real chance,” he said.

It was Tiafoe’s return, an area of his game that he has augmented with new coach David Witt, that took the match off Popyrin’s racket in the early stages. The huge-serving Shelton said on Friday that Tiafoe had probably returned better against him than anyone all year. Tiafoe broke his compatriot five times in five sets in that match. He went after the serve of Popyrin, also a huge weapon, with similar purpose.

He clinched the decisive break of the first set at 4-4 with some clutch returning, including on the second point of the game, when he stretched out to wide to retrieve a 118 mph delivery that looked to be careering towards the match clock. In the second set, Tiafoe looked out of it down 5-3, 40-0 on the Popyrin serve, but he miraculously manufactured a break out of nowhere.

Popyrin helped him out with a couple of errors, but a high backhand return winner on the last of three set points underlined how well Tiafoe was reading his opponent’s serve, and how effectively he was using its speed to his advantage.

Buoyed by that reprieve and with his opponent rattled, Tiafoe raced through the subsequent tiebreak to open up a two-set lead. He played with such authority and creativity that it brought to mind the 7-0 tiebreak beatdown he served Andrey Rublev on this court, in the quarterfinal two years ago. After an ace wrapped it up, the crowd, a formidable force for Tiafoe here, were on their feet again. After the match, Tiafoe, 26, seeded No 20 here, said he’s learned how to pick his moments to harness the crowd’s energy. “I’ve been figuring it out much more,” he said after the match.

Popyrin said afterwards that he “choked” away the second set, but he reset impressively. Having looked blunted for much of the match following his exploits two nights ago, the No. 28 seed won the third set 6-2, quietening the crowd by changing his tactics. His first-serve points won percentage also shot up, from 72 in the second set to 93 in the third, as Tiafoe suddenly struggled for rhythm.

Now it was the American who had to regroup, and like his return, he feels he’s improved his reset strategy considerably.

Alexei Popyrin’s serve, which propelled him to victory over Novak Djokovic, fell flat on Sunday night. (Charly Triballeau / AFP via Getty Images)

“I think it’s been something I have been doing much better,” he said after beating Shelton.

“Having pride in myself, just don’t lay down. I just want to win or lose matches, knowing the guy beat me; I didn’t beat myself. No free lunches. I’m not trying to help my opponent get over the line.”

Tiafoe added that he’s been working with “a mental guy, process guy” on this aspect of the game.

At the start of the fourth set against Popyrin, Tiafoe held on. He was 0-30 down at 1-1, but dug out the hold as the crowd went wild, even more so when Tiafoe changed his shirt at 2-1. Three games later, Tiafoe had the break he needed, as a sliced backhand return on the stretch, and then a clever readjustment of his body to deal with a serve kicking towards his upper torso drew Popyrin errors. All of a sudden it was 4-2, and Popyrin finally faded out.

Tiafoe served the match out, producing a stupendous drop volley in the final game, before Popyrin missed a forehand in the tape that would have taken the score to 5-4, and Tiafoe spun the ball on to the white paint one last time.

From underdog to the land of possibility, in the space of an invigorating couple of days.

(Top photo: Charly Triballeau / AFP via Getty Images)