Defending US Open champion Rafael Nadal sent close friend David Ferrer into grand slam retirement on Monday when his veteran compatriot was forced to quit their first-round match with injury, but former winners Andy Murray and Stan Wawrinka made successful returns to New York.
Ferrer, a former world No 3 but now standing at a lowly 148, called it quits on a hot and humid night on Arthur Ashe Stadium despite leading 4-3 with a break in the second set after twice receiving treatment on a left leg injury.
Top seed Nadal had taken the opener 6-3.
"This is my last grand slam," said Ferrer who was runner-up to Nadal in the 2013 French Open final.
"I have great memories of this tournament and I am just sorry that I couldn't finish it tonight."
Ferrer, 36, will play his last tournament on home ground in Madrid next year.
"I am very sorry for David and sad for him," said Nadal. "We are close friends and have shared great moments - the Roland Garros final and Davis Cup wins.
"He is one of the greatest players to have come from our country. He's a great person and great player."
Nadal, chasing his 18th grand slam title, will face Canada's Vasek Pospisil for a place in the last 32.
Nadal's potential semi-final opponent, Argentine third seed and 2009 champion Juan Martin del Potro, also reached the second round with a 6-0, 6-3, 6-4 win over American qualifier Donald Young.
For Del Potro, whose career since his maiden grand slam triumph nine years ago has been decimated by wrist problems, it was his 30th career win at the tournament.
He next faces Denis Kudla of the United States who won his first match in five years at the tournament when he downed Matteo Berrettini of Italy 6-4, 7-5, 6-2.
Former world No 1 Murray marked his first grand slam tournament in 14 months with a 6-7, 6-3, 7-5, 6-3 win over Australia's James Duckworth.
Murray, the 2012 champion and playing just his fifth tournament of the year after undergoing hip surgery in January, last featured in a major at Wimbledon in 2017.
Murray, whose world ranking has slumped to 382, goes on to face Fernando Verdasco, the Spanish 31st seed after seeing off the error-prone Duckworth, ranked at 448.
"He was serving big in the first set and the ball was flying around," said 31-year-old Murray who only returned to the tour in June.
"But I started to get into the return games, stood back a little more and that gave me more time to get in the rallies and find my range with my ground strokes."
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Like Murray, former champion Wawrinka also sat out the 2017 US Open - in his case because of two knee surgeries.
But he showed no sign of rustiness on Monday, defeating eighth-seeded Grigor Dimitrov of Bulgaria for the second successive grand slam.
Wawrinka, the 2016 champion, swept to a 6-3, 6-2, 7-5 victory on Arthur Ashe Stadium to repeat his opening round triumph over Dimitrov at Wimbledon.
"The last time I played on this great court, I won the title so it was great to be able to come back and play again," said the 33-year-old Swiss.
Wawrinka, the former world No 3, now down at 101, shrugged off having to take a medical time-out early in the third set.
South African fifth seed Kevin Anderson, the 2017 US Open runner-up to Nadal, also needed a medical timeout for a leg injury in the third set of his clash with Ryan Harrison of the United States.
But Anderson, defeated in the Wimbledon final last month by Novak Djokovic, recovered to win 7-6, 5-7, 4-6, 6-3, 6-4.
Denis Shapovalov, the 19-year-old 28th seed, was 7-5, 5-7, 4-1 ahead of 18-year-old Felix Auger-Aliassime when his Canadian teammate and close friend was forced to retire.
Auger-Aliassime, making his slam debut after coming through qualifying, had needed a medical timeout for an irregular heartbeat in the third set.
Greek 15th seed Stefanos Tsitsipas saw off 36-year-old Tommy Robredo of Spain 6-3, 7-6 (7/1), 6-4.
Tsitsipas, 20, made the Toronto Masters final this month seeing off the likes of Djokovic and Alexander Zverev on the way before losing to Nadal.
The Greek was only three years old when Robredo played his first US Open in 2001.
US hopes were buoyed by wins for 11th seed John Isner and 18th-ranked Jack Sock who won his first singles match since May.