Williams, often considered the best women’s tennis player in history and Spain’s seven-time Grand Slam singles champion Alcaraz fell to Belinda Bencic of Switzerland and Flavio Cobolli of Italy, 5-4, 4-1.
The result means Bencic and Cobolli will advance to Wednesday’s finale of the unorthodox competition, when the semi-finals and finals will be played back-to-back.
“We felt their aura for sure,” Bencic told a television interviewer afterward. “It was an honour to play.”
“Playing Serena and Carlos on Arthur Ashe is not easy,” Cobolli said. “So now maybe we take a beer.”
Williams and Alcaraz had shown strong form earlier defeating the British-New Zealand pair of Lloyd Glasspool and Emma Routliffe.
But Cobolli, who is having a breakout season in singles as the French Open runner-up and Bencic, an Olympic Gold medalist with singles wins over Williams, proved too much.
Williams was making her competitive return to Arthur Ashe Stadium since her 2022 retirement, paired with Spain’s Alcaraz, himself back after a lengthy injury layoff.
The pair, dressed in coordinated purple and black jersey-type outfits, smiled as they bantered and joked on changeovers. The audience, which roared their arrival in Tuesday’s first round, was a bit more subdued in the second match.
Earlier on Tuesday, another high-wattage pairing, former world number one Novak Djokovic and four-time Grand Slam champion Aryna Sabalenka also were sent packing, losing to Czech Katerina Siniakova and Britain’s Henry Patten in three sets.
The US Open mixed doubles format features a blend of doubles specialists and star singles players pairing up, sometimes with a bit an experimental mindset.
The event, in its second year, will have a new winner after defending champions Andrea Vavassori and Sara Errani, both doubles specialists, lost to the Russian pair of Mirra Andreeva and Andrey Rublev 1-4, 4-1, 11-9.
The Czech pairing of Karolina Muchova and Jakub Mensik, both highly ranked singles players, realised quickly they had to adjust when they faced Kristina Mladenovic of France and Marcelo Arevalo of El Salvador, both of whom have won Grand Slam doubles titles.
“They keep us more on our toes,” Muchova said after a quarterfinal win put the pair in the semifinals. “They see the court I would say a little bit different.”
One first-time team that scored unexpected success was the husband-and-wife pairing of France’s Gael Monfils and Ukraine’s Elina Svitolina.
They won two matches in the shortened format, including a tightly contested quarterfinal win over Siniakova and Patten 5-4, 4-5 and 10-8 in the decisive tie-break.
“We had zero expectations,” Svitolina said after the match.
The pair only learned late that they had been granted a wildcard and had joked about not winning a game.
Monfils, who will retire after this season, said it is still too early for talk about winning the title, but “we will have great images for our memory.” (AFP)