NEW YORK — On the fifth day, the king of tennis left the building.

Twenty-four hours after Carlos Alcaraz was knocked out of the US Open, Novak Djokovic, the reigning champion and 24-time Grand Slam winner, followed him out. The hard-serving and suddenly deadly Australian Alexei Popyrin put Djokovic in the scrap heap of the men’s seeds at this year’s tournament, winning 6-4, 6-4, 2-6, 6-4.

While No. 1 Jannik Sinner still stands, these two results in two nights have blown up the men’s US Open field. Grand Slam near-misses like Casper Ruud and Alexander Zverev, and a group of hometown dreamers desperate to become the first American man to win a Grand Slam since 2003, are suddenly alive with a clearer path.

Djokovic’s loss was completely explainable and downright astonishing at the same time.

He is 37, two and a half months removed from knee surgery. A month ago, he achieved perhaps the most emotional triumph of his career, a stunning victory over Alcaraz to capture Olympic gold, the only major tennis prize that had eluded him.

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He played that tournament at the Paris Games on the clay of Roland Garros. He did not play a match on hardcourt until his first victory at this year’s tournament in New York, Monday night against Moldovan Radu Albot. The victory at the Olympics — his first title of the year — gave him new energy, but he still said he had achieved everything he wanted to in his career.

As a tennis player he was complete.

Novak Djokovic struggled to get going against a noisy Arthur Ashe. (Sarah Stier/Getty Images)

A loss so early in a Grand Slam tournament still comes as a shock. Djokovic hadn’t lost so early in a Grand Slam since 2017, when Denis Istomin knocked him out at the Australian Open. That’s when his right elbow began to deteriorate and he eventually needed surgery.

Over the past four years, only a freak injury, a foul for unintentionally hitting a linesman with a ball and his refusal to be vaccinated against the coronavirus have kept Djokovic out of Grand Slam tournaments.

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On Friday night, Popyrin stood with his loose arm in front of a jubilant crowd of more than 24,000 people at Arthur Ashe Stadium.

He had already come ahead of Djokovic twice before in Grand Slams this year. He was one point away from a two-sets-to-one lead in the second round at the Australian Open in January. Then he took a set from a handicapped Djokovic at Wimbledon before Djokovic’s strength on grass became too much for him.

Friday night looked set to be a more routine evening for Djokovic. The stifling heat and humidity that had turned Ashe into a steam bath on Wednesday had broken, giving Djokovic a chance to play in the cooler evening air where he normally thrives. He continued to use a cooling tube and mitten during the changeovers.

He seemed only half-interested in the fight during his first few matches. Motivation was sputtering. But then Alcaraz lost, and Djokovic’s path to the title suddenly became clearer. It’s the kind of development that has fueled him in the past. It brought a record-breaking 25th Grand Slam title that little bit closer to his grasp, even at the end of a gruelling summer spent battling his aging body.

By the time he took notice of Popyrin, the lean, 6-foot-5 Australian with shoulders as broad as letterboxes had fought his way to a two-set lead, intimidating the greatest returner in the history of the sport with a serve that ducked into corners and took off, allowing Popyrin to easily score second balls with his long drives.

Alexei Popyrin has played against Novak Djokovic three times this year at Grand Slam tournaments. This time he won. (Angela Weiss/AFP via Getty Images)

Djokovic had been in this position so many times. Eight times he had come back from two sets down at a major. Last year he did it at this tournament, against his compatriot Laslo Djere.

Usually, all he needs is a service break in the third set to get going. He wins it, and the man on the other side of the net is gone before the relentless surge he knows is coming overwhelms him.

He got that on Friday night against Popyrin and ran away with the third set as the Australian suffered a sprained ankle that limited his range of motion. Djokovic raised his arms to the crowd and urged them to be loud for him, but there was something half-hearted about it, the warrior look in his eyes not really there.

In the fourth set, Popyrin did what Djokovic’s victims never do in these matches. He rediscovered his spine. In the fifth game, pinned three feet behind the baseline, he unleashed a ferocious inside-out forehand that stuck to the corner of the court like metal to a magnet.

Djokovic had no choice but to turn his head and watch as the ball hit the target and the stadium exploded. Popyrin got his first glimpse of the finish line. Normally, Djokovic would pull the ball back into the distance. Instead, Djokovic pushed Popyrin much closer two games later. A series of double faults, punctuated by two brilliant passes from Popyrin and a forehand over the baseline, brought the match to 5-2.

Djokovic stood by his chair for a moment during the changing of the guard, then walked to the back of the court, like a man ready to get his execution over with.

The tactic worked: Popyrin had a weak match and Djokovic got a reprieve.

A match later, Popyrin found his backbone again. An ace on the second serve gave him three match points, then Djokovic’s final forehand went long, ending the night and blowing up the tournament.

(Top photo: Sarah Stier/Getty Images)