NEW YORK — The U.S. Open may be lacking star wattage with former champions Roger Federer, Rafael Nadal and Serena Williams sidelined by injuries, but it kicked off Monday with the glitz and glamour of a Broadway show.

More than 53,000 tennis fans flocked to the grounds after being barred by the coronavirus pandemic last year.

“It has been a tough 18 months, but we have shown the world nothing is going to beat us,” equality advocate and former champion Billie Jean King, for whom the U.S. Tennis Association’s National Tennis Center is named, said in welcoming the night-session crowd. “Look out world, New York City is back!”

Earlier, the late-morning crush of arrivals at the Flushing Meadows venue caused a lengthy backup at the east gate, where fans streaming in from the nearest subway stop were forced to wait nearly an hour to enter.

Tournament officials vowed to do better and said the backlog was caused by the bag-check process rather than the requirement given on short notice that fans must show proof of at least one dose of a coronavirus vaccine to enter.

Once on the grounds, fans’ celebration of tennis was in full swing.

Few bothered with mask-wearing or social distancing as they ate, drank and cheered with gusto as the players went to work in pursuit of the season’s final major in the absence of Williams, Federer and Nadal, who boast 63 Grand Slam singles titles among them.

Two-time and defending champion Naomi Osaka, who headlined the night session at Arthur Ashe Stadium, leaned on her powerful serve to subdue Marie Bouzkova of the Czech Republic, 6-4, 6-1.

Osaka, who was cheered by the crowd in her first Grand Slam match since withdrawing from the French Open this spring to care for her mental health, thanked fans for attending.

“Last year, when we didn’t have the crowd, I know it felt quite lonely for me,” Osaka said during an on-court interview, and fans stood and applauded. “It’s great to see little kids and, of course, grown-ups. I think the energy here is definitely unmatched.”

French Open finalist Stefanos Tsitsipas, 23, edged former world No. 1 Andy Murray in a five-set ordeal that left the Scot seething after Tsitsipas continued a recent pattern of taking medical timeouts and extended bathroom breaks at critical junctures in close contests.

Murray said afterward that he had prepared mentally for the tactic but couldn’t prepare his body, at 34, for the effect of halting all exertion for several minutes amid a punishing, nearly five-hour match.

“I’m not saying I necessarily win that match, for sure, but it had influence on what was happening after those breaks,” Murray said, adding that he felt Tsitsipas was “a brilliant player” and great for tennis.

“But I have zero time for that stuff at all,” Murray said, “and I lost respect for him.”

American teen Coco Gauff exulted in fans’ return after weathering a three-set challenge from Poland’s Magda Linette.

“It’s crazy what support can do,” said Gauff, 17. “People believe in you when you don’t believe in yourself; that can really change the way a match goes.”

As U.S. Open tournament director Stacey Allaster put it in an interview Monday, the return of fans simply quickened the pulse of the event.

“They feed the energy of the U.S. Open,” Allaster said.

Their spending also augments broadcast revenue that funds the U.S. Tennis Association’s mission of promoting and growing the sport, she noted.

Fans don’t just account for two weeks of ticket revenue. They also represent value to the U.S. Open sponsors that use the event to showcase their goods and services, such as Emirates airline, Tiffany & Co. jewelry or Rolex watches.

In 2020, the loss of ticket sales, hospitality suites and other revenue sources was projected to pare the U.S. Open’s net income roughly 80 percent, USTA chief executive Mike Dowse told the Associated Press.

As players toiled on the courts Monday, the wheels of commerce turned on nearly every square foot of the grounds. Business was brisk at the Baseline Cocktails kiosk, where lines formed before noon for the U.S. Open’s “signature cocktail,” the Grey Goose Honey Deuce, at $20 a cup. Other kiosks sold the U.S. Open’s official water, official beer, official coffee and official sports drink.

At the display of the tournament’s official vehicle, Mercedes-Benz, no driver’s license was required to sit behind the wheel of a fully electric 2022 EQS.

All of this commerce has a trickle-down effect on players.

U.S. Open officials last week announced a record $57.5 million in prize money and compensation, which includes a reallocation that shifts a portion of the top players’ winnings to that of also-rans to address an income inequity that was highlighted by the pandemic’s challenges.

Under the new prize structure, brokered by the USTA and the men’s and women’s pro tours, the men’s and women’s champions will take a pay cut, from $3 million to $2.5 million, while those who competed in the qualifying tournament and all first-round participants will get an increase.

But the biggest pretournament adjustment had to do with coronavirus protocols for spectators.

U.S. Open officials announced last week that fans would not be required to show proof of vaccination or a negative test, given that the tournament is an open-air event. That policy was approved by both New York state and city health department officials, Allaster said.

Within 48 hours, New York Mayor Bill de Blasio announced an “overrule,” demanding stricter protocols for spectators at Arthur Ashe Stadium, the biggest venue on the grounds, given that when its retractable roof is deployed, it becomes an indoor venue.

Rather than create two sets of protocols based on ticket location, tournament officials announced Friday that all spectators would be required to show proof of at least one dose of a vaccine.

Americans Sloane Stephens and Madison Keys were first on Arthur Ashe Stadium on Monday in a reprise of the 2017 women’s singles final. Stephens got the victory again but by the narrowest of margins, prevailing in a third-set tiebreaker.

The two are good friends and have known each other since they were 12, so it’s a matchup neither looks forward to, Stephens explained afterward, particularly at Grand Slams.

“It’s just tough when you get into this position, and we’re at the U.S. Open; we’re both looking to do well here,” Stephens said. “And then to play each other. … Someone has to win it, someone has to lose, and it’s just sucky.”

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