Dominic Thiem, of Austria, returns a shot to Alexander Zverev, of Germany, during the men’s singles final of the US Open tennis championships, Sunday, Sept. 13, 2020, in New York. (AP Photo/Seth Wenig)

NEW YORK — At a U.S. Open unlike any other, Dominic Thiem constructed a comeback the likes of which hadn’t been seen in 71 years.

After dropping the opening two sets against Alexander Zverev on Sunday at a nearly empty Arthur Ashe Stadium — fans were banned because of the coronavirus pandemic — Thiem slowly but surely turned things around for a 2-6, 4-6, 6-4, 6-3, 7-6 (6) victory across more than four hours to earn his first major championship.

The 27-year-old from Austria is the first man to win the American Grand Slam tournament after trailing 2-0 in sets in the final since Pancho Gonzalez did it against Ted Schroeder in 1949 at an event then known as the U.S. Championships.

Not only that, but in a fitting finish to an unprecedented two weeks, this match was decided by a fifth-set tiebreaker, something that had never happened in the final.

When it ended on a groundstroke flubbed by Zverev, a weary Thiem collapsed on his back way behind the baseline. Zverev — who himself came within two points of the victory — walked around the net to offer a handshake and hug to his pal, two gestures rarely spotted in this era of social distancing.

Thiem had come in 0-3 in Grand Slam finals, but always came up against Rafael Nadal or Novak Djokovic in those others.

This time, he was the favorite and came out jittery, but eventually worked his way out of that, while Zverev went from cool and confident to passive and pushed around.

The fifth set was just as back-and-forth as the other four, the mistakes rising with the tension and the history in the offing.

Thiem broke in the opening game when Zverev shanked a pair of forehands.

Zverev broke right back — and pierced the silence with a rare cry of “Come on!” — when Thiem double-faulted.

Then it was Zverev’s turn to nose ahead, breaking for a 5-3 lead when Thiem sent a down-the-line backhand wide and leaned over, gasping for air.

But with a chance to serve out the biggest win of his nascent career, Zverev faltered, getting broken right back when he pushed a volley into the net.

That began a three-game run for Thiem, who broke to lead 6-5, earning his own chance to serve for it, when Zverev netted a backhand, followed by a long forehand.

A trainer came out to check on Thiem’s right leg during the ensuing changeover. He couldn’t seal the deal then, either, and eventually needed a trio of match points to end it.

While this was the No. 7-ranked Zverev’s first Slam final, this was the first one that Thiem was supposed to win, following losses to Nadal at the French Open in 2018 and 2019, then to Djokovic at the Australian Open this February — back before the pandemic upended the world and prompted a U.S. Open with zero fans and regular COVID-19 testing.

Instead of wild applause and loud shouts greeting great exchanges, the soundtrack at Arthur Ashe Stadium mainly came from outside the largest court in tennis, courtesy of roaring airplanes, rumbling trains, revving car engines, honking horns and wailing sirens. There was the occasional polite applause from the dozens of tournament workers allowed in the stands — and, deep into the match, yells from the players’ entourages.

But the louder crowd noise heard by TV viewers was fake, added by the broadcaster.

Unable to draw from support in what’s always been an electric environment, on an evening that felt more like a glorified practice session than a match with so much at stake, both men were sluggish at times, listeless, even.

Normally, the U.S. Open closes each Grand Slam season, but what about 2020 has been normal?

“Strange times,” Thiem called it.

Because of the COVID-19 outbreak, Wimbledon was canceled for the first time since World War II, while the French Open was postponed from its originally scheduled May start and now will begin in two weeks.

So the tennis world quickly shifts to European clay after an unusual two-tournament hard-court doubleheader in Flushing Meadows — called “a crazy idea” by U.S. Tennis Association CEO Mike Dowse on Sunday.

The U.S. Open was preceded at its site by the Western & Southern Open, which moved from Ohio to New York as part of a “controlled environment” to limit travel.

Another way in which this whole event was different: A member of the Big Three of men’s tennis — Roger Federer, Nadal or Djokovic — had won the preceding 13 major trophies.

So Thiem took advantage of the chance to sneak into the club of champions.