NEW YORK — After four hours and 40 minutes of tennis, after Frances Tiafoe watched one last forehand passing shot curl off his strings and down the line, there was only one thing left to do.Tiafoe sprinted forward — not to his team, but to Alex Michelsen, who fell to weeping in his arms. The 21-year-old Californian is one of Tiafoe’s tennis brothers. He wrapped Michelsen in a massive bear hug, consoling him as Michelsen buried his face in Tiafoe’s neck and shook with tears.Tiafoe didn’t let him go. He’s been there so many times the past two years, including on this court in 2024, when he gave up a lead in the semifinal against Taylor Fritz. On Tuesday afternoon, and into the evening, he was on the other side of that cruel tennis equation.With Michelsen, the big-serving talent, on the verge of the biggest win of his career, Tiafoe grabbed hold of a lifeline from a wobbling opponent and improbably climbed all the way back from two sets and 5-3 down to win 5-7, 3-6, 7-5, 6-3, 7-6(10-6) and reach the U.S. Open last four, where he will face Ben Shelton, conqueror of Carlos Alcaraz.“It’s the quarterfinals of a Slam, you got to leave it out there,” Tiafoe said.That much he did.Tiafoe was gone, then he was back, then he seemed gone again. Then, he rose once more, to head off a last charge from Michelsen with a flurry of aces and nasty slices that had seemed so far beyond his abilities just a couple hours before.Over the last seven games, the match became everything that it seemed to promise it when the day started. Tiafoe and Michelsen cracked serves and charged the net, and feathered drop volleys and found angles and seams in the court that only they saw.When it counted most, Tiafoe found a couple more of them. He cracked a forehand down the line for an early edge in the tiebreak, then kicked a second serve beyond the useful range of even Michelsen’s long reach. Michelsen would get even, but then Tiafoe blasted a forehand return across the court for a winner. It gave him the most precious lead: the one he never gave back.Tiafoe raced across the finish line, with an ace, another unreturnable second serve, and then that curling winner down the line that he has probably hit 10,000 times since he picked up a tennis racket, but never when it counted like this.And then he had Michelsen in his arms, trying to keep his emotions in check and failing at it in the best way, as Tiafoe gave him the words that he’s gotten too many times before.“He was saying a lot of, you know, nice personal things, you know, you’ll be there, when I’m older, you’re going to be here, and we’re going play again in this type of circumstance and all that good stuff,” Michelsen said in a news conference later.“He’s a great guy for consoling me in that moment.”“I really like Alex, I really, really do, and I just wanted to come up there and just dap him up,” Tiafoe said in his news conference. I didn’t think it was going to be as long as it was, but I understand the moment.”For more than two hours and nearly three sets, Tiafoe had been one of the worst versions of his tennis self. In the second set, he landed just 35 percent of his first serves, and Michelsen was mostly crushing him on his second serves. For a player who rises for his home Grand Slam and puts on a show like few others, it was awkward.