NEW YORK — After five-and-a-half hours of opening acts, the main event of the first day of the U.S.Open got underway when Serena Williams and Carlos Alcaraz walked onto the court at Arthur Ashe Stadium.There was just a bit of noise. The sort of noise that only rises through stadiums for generational talents who just happen to have been away from a sport for longer than any of that sport’s fans might have liked — four months for Alcaraz, and the better part of four years for Williams.It rattled around the steel and concrete of the biggest stadium in tennis in a way that becomes something felt instead of heard. A point into their match, Williams closed in on a floating return and smashed it through the court, letting out one of those patented grunts, and off they went.Williams, 44, a 23-time Grand Slam singles champion and 16-time Grand Slam doubles champion, the GOAT of women’s tennis to many, is enjoying one of those last rides in the sunset that few expected, especially her.How do fans feel about Serena Williams' comebackLauren Morales-Jones and Johnny SweetHow long will it last? Days? Into next season? No one really knows, though she swears it won’t be long. Williams did not take a wild card into the singles competition, leaving women’s doubles with her sister Venus, with whom she won 14 of those Grand Slam doubles titles, as her only other event at this tournamentAlcaraz, 23, is still in the early years of what should be an epic career, though a worrisome wrist injury has him and much of the sport wondering what his immediate future will look like. He has seven Grand Slam singles titles already, including one of each, the youngest man ever to accomplish that.How’s that for a magnetic pairing? And those screams. What a moment.It got better. Down 3-1 in the first set against Erin Routliffe and Lloyd Glasspool, two seasoned doubles players, Alcaraz and Williams came alive to reel off eight of the next nine games, two all-time greats shedding the rust and riding pure talent and instinct to overcome and overpower rivals who know the geometry of the doubles court much better than them, but did not know what had hit them. Routliffe maybe knew a little too well, having faced Williams thrice this summer with one win to show for it.There was Alcaraz doing those crazy Alcaraz things — the lunging miracle touch volleys, the impossible gets, the smile lighting up the building. There was Williams, rocketing aces and finding angles that looked impossible until the ball left her strings and curved exactly where she asked it to go.“She made it really easy for me,” Alcaraz said on the court after a 5-3, 4-1 win in this fast-four format, the second edition of the U.S. Open’s mixed doubles that was redesigned to favor singles star power — and to bring in a much larger check for the winners — in 2025.Williams was equally effusive.“This is such a bonus,” she said. “To play with Carlos is honestly so cool.”Gaël Monfils and Elina Svitolina brought more joy to the crowds at the U.S. Open during Fan Week. (Timothy A. Clary / AFP via Getty Images)There was plenty that Alcaraz couldn’t yet do as he returns from his right wrist injury. His serve speeds were way off their peak. He looked rusty. What his time on court with Williams said about his quest to defend his singles title likely won’t be legible until he plays his first-round match next week.Elsewhere, it was another test of concept for the U.S. Open’s choice to remake a competition and, in so doing, alienate the people who have dedicated their careers to playing it.Last year, after being maligned out of the competition that they had won in the previous one, Italy’s Sara Errani and Andrea Vavassori knocked down a clutch of boldface singles names to win the title and take home $1 million. This year, after a qualifying event brought even more bona fide doubles teams into the competition, including world No. 1s Kateřina Kateřina Siniaková and Henry Patten, such an outcome looked likely again.It turned out, not so much. The semifinals could have been all doubles specialists. In the end, they were all singles. Siniaková and Patten knocked off Aryna Sabalenka and Novak Djokovic with consummate ease after a loose opening set, but Gaël Monfils and Elina Svitolina, the husband-and-wife duo, edged them out in a 10-point deciding tiebreak. Routliffe and Glasspool fell to Alcaraz and Williams. And Errani and Vavassori lost a match tiebreak of their own to Mirra Andreeva and Andrey Rublev.Come Wednesday, it will be Svitolina and Monfils against Belinda Bencic and Flavio Cobolli, the BNP Paribas Open at Indian Wells mixed doubles champions who toppled Alcaraz and Williams. Andreeva and Rublev will play Jakub Menšík and Karolína Muchová, who just might be the tennis favorites, with Menšík’s lethal serve and Muchová’s gossamer touch.But for vibes, on a day that was full of them? Monfils and Svitolina, who lit up the crowd from start to finish, are undefeated among the last four.“We understand each other by the look,” Svitolina said. “we can find the right words for each other.”Mixed doubles can be an odd discipline, all about quickness and angles and surprise. But vibes, of every kind, can go a long way too.