Alcaraz arrived at the United States Tennis Association Billie Jean King National Tennis Center seeking back-to-back Grand Slams after his Wimbledon triumph last month, and started against world number 70 Koepfer on Arthur Ashe Stadium.

Koepfer rolled his ankle in the opening game which left him hampered as he lost the first set 6-2, and retired trailing 3-2 in the second.

After a shortened start to the final tennis Grand Slam of the year, Alcaraz is set to face South Africa's Lloyd Harris in the second round.

"It was a shame for him in the first game, he got injured with the ankle," the 20-year-old Alcaraz said.

"Obviously that's not the best way to get through to another round.

"But I have to say for myself that I was playing great, I was playing really well on court.

"Hopefully I can keep the same level in the next round."

A player has not defended the men's singles title at the US Open since Switzerland's Roger Federer's five consecutive victories from 2004 to 2008.

Another recent winner in 2021 champion Daniil Medvedev, a Russian player competing as a neutral due to the war in Ukraine, dropped just two games in a 6-1, 6-1, 6-0 victory against Hungary's Attila Balázs, which took just 74 minutes to wrap up.

The other two former winners in the draw - 36-year-old Briton Sir Andy Murray and 38-year-old Stan Wawrinka of Switzerland - are also through to the second round.

Sir Andy, the 2012 winner, overcame Corentin Moutet of France 6-2, 7-5, 6-3, and 2016 champion Wawrinka beat Japan's Yoshihito Nishioka 7-6, 6-2, 6-4, both on Grandstand.

Sir Andy is up to 200 career Grand Slam victories.

The video-assist review system featuring at the US Open for the first time was requested by Moutet for a double bounce called against him in the final game, which led to a lengthy delay before umpire Louise Engzell declared the technology was "not working".

Sir Andy claimed he was in favour of using technology but branded the situation "a bit of a farce".

Olympic champion Alexander Zverev of Germany continued his return to form from a nasty ankle ligament injury sustained in last year's French Open semi-finals as he won all three sets 6-4 against Australian debutant Aleksandar Vukic.

Tokyo 2020 silver medallist Karen Khachanov, a Russian competing as a neutral, was among the high-profile exits as he went down 6-2, 6-4, 6-2 to American wildcard Michael Mmoh.

In the women's singles, Belarusian neutral Aryna Sabalenka, seeded second as she seeks a second Grand Slam following her Australian Open victory earlier this year, started with a 6-3, 6-2 win against Belgium's retiring Maryna Zanevska.

Ukrainian-born Zanevska refused to shake Sabalenka's hand after the match, a stance taken by several of her compatriots against Russian and Belarusian opponents since the start of the war in February 2022 but which drew a smattering of boos from the crowd on Louis Armstrong Stadium.

Sabalenka is set to face Britain's Jodie Burrage in the second round.

Third seed Jessica Pegula, the ighest-ranked American in the tournament, triumphed 6-2, 6-2 against Italy's Camila Giorgi, but that match was followed by her compatriot Venus Williams - the 2000 and 2001 US Open winner competing as a wildcard - suffering her biggest-ever defeat at the tournament 6-1, 6-1 to Belgium's Greet Minnen.

Seventh seed Caroline Garcia of France, a semi-finalist at Flushing Meadows last year, suffered a first-round exit 6-4, 6-1 to China's Wang Yafan.

Tunisia's Ons Jabeur, a finalist in 2022 who lost the Wimbledon final for the second year running to extend her wait for a maiden Grand Slam admitted she considered retiring on Louis Armstrong Stadium after requiring medical treatment in the first set and having her blood pressure taken during a timeout.

However, she progressed 7-5, 7-6 against Colombia's Camila Osorio.

Second round matches in both singles draws are due to start tomorrow.