Carlos Alcaraz launched the defence of his US Open crown with a brisk victory after his opponent suffered an injury on Tuesday as third seed Daniil Medvedev strolled into the second round with a quick-fire win.
Alcaraz, returning to the same Arthur Ashe Stadium court where he lifted his first Grand Slam as a teenager a year ago, was always in control after Germany's Dominik Koepfer rolled his left ankle badly in the opening game.
Koepfer tried to continue despite being in obvious discomfort, but finally called it quits while trailing 3-2 in the second set after dropping the first 6-2.
"That's not the best way to get through into another round," said world number one Alcaraz. "But I have to say on my side I was playing great."
Alcaraz, who faces Lloyd Harris of South Africa in the second round, is on course to face 2021 US Open champion Medvedev in the semi-finals.
Medvedev opened his account on Tuesday with a 6-1, 6-1, 6-0 thrashing of Hungary's Attila Balazs in just 74 minutes.
The Russian will face Australia's Christopher O'Connell in the second round on Thursday for a spot in the last 32.
"That's probably the fastest three-set match I played," Medvedev said after his blistering demolition of Balazs.
While Alcaraz and Medvedev cruised into the second round, Karen Khachanov -- a semi-finalist at last year's US Open -- was on the wrong end of a first-round upset.
The 11th-seeded Russian was bounced out in straight sets by American world No.89 Michael Mmoh, losing 6-2, 6-4, 6-2.
Germany's 12th seed Alexander Zverev, a beaten finalist at Flushing Meadows in 2020, never looked in danger of following Khachanov out of the tournament, registering a 6-4, 6-4, 6-4 win over Australia's Aleksandar Vukic.
Jannik Sinner, the Italian sixth seed, trounced Yannick Hanfmann of Germany 6-3, 6-1, 6-1.
In other men's games on Tuesday, British veteran Andy Murray downed France's Corentin Moutet 6-2, 7-5, 6-3, in a match that saw a bungled attempt to use the US Open's new video review technology for the first time.
Chair umpire Louise Azemar Engzell attempted to deploy the system after Moutet requested a review following a double-bounce call.
The umpire was forced to abandon the attempt however after discovering officials were unable to properly access footage of the incident.
"I don't know how the technology works," Murray said. "But it obviously didn't go to plan in a pretty important moment of the match. So, yeah, it would be good if they could get that fixed."
Meanwhile, China sealed a notable first when Wu Yibing beat Serbia's Dusan Lajovic 3-6, 6-4, 2-6, 6-4, 6-2 to join compatriot Zhang Zhizhen in the second round.
It is the first time in tennis's Open era that two Chinese men have reached the second round of the same Grand Slam tournament.
- Ailing Jabeur advances -
In the women's draw meanwhile another semi-finalist from 2022 also made an early exit, with French seventh seed Caroline Garcia slumping to a 6-4, 6-1 reverse against Chinese qualifier Wang Yafan.
Meanwhile second seed Aryna Sabalenka of Belarus, the reigning Australian Open champion, progressed with a 6-3, 6-2 win over Ukraine-born Belgian Maryna Zanevska.
The end of the match was tinged with controversy as Zanevska declined to shake hands with Sabalenka in protest at Belarus's military support of Russia's invasion of Ukraine.
"I know I'm representing Belgium but all my family are in Ukraine," Zanevska told AFP.
"I have family fighting on the frontline. And I think if I shake her hand it would be the same as spitting in the face of them."
Tunisia's Ons Jabeur, desperate to land her first Grand Slam title after losing in three major championship finals in 2022 and 2023, survived a medical scare before scraping past Colombia's Camila Osorio.
The 29-year-old fifth seed looked out of sorts throughout her 7-5, 7-6 (7/4) win, at one stage in the opening set complaining of breathing difficulties before taking an extended medical timeout.
Jabeur has played only one tournament since being upset in last month's Wimbledon final by unseeded Czech Marketa Vondrousova.
Vondrousova, seeded ninth in New York, cruised into the second round on Tuesday with a 6-3, 6-0 win over South Korean qualifier Han Na-Lae.
Third seed Jessica Pegula meanwhile kept home hopes alive with a clinical 6-2, 6-2 defeat of Italy's Camila Giorgi.
But there was disappointment for American icon Venus Williams, competing in her record-extending 24th US Open.
Williams was bounced out 6-1, 6-1 by Belgium's Greet Minnen -- who was just a few weeks old when Williams reached her first US Open final back in 1997.