Rafael Nadal's first match since pulling out of the Wimbledon semi-finals because of injury ended in defeat at the hands of Croatian Borna Coric at the Cincinnati Open.
The Spaniard, 36, lost 7-6 (9-7) 4-6 6-3 to the world number 152.
Nadal has spent six weeks recovering from the abdominal muscle injury and the defeat in the round-of-32 match is a blow to his US Open preparations.
The year's final Grand Slam event gets under way on 29 August.
"In terms of New York, it's a Grand Slam, a different kind of tournament," said Nadal, who has won the US Open in 2010, 2013, 2017 and 2019.
"I was only able to play two sets before this match in the last 40 days.
"I know that to gain condition. I will have to be on the tour practising with the guys."
American Taylor Fritz, who Nadal beat in the Wimbledon quarter-finals, defeated Australian Nick Kyrgios 6-3 6-2 to reach the third round.
Fritz then criticised the ATP Tour's off-court coaching trial, saying allowing players to receive instructions during a match went against the idea of tennis being an individual sport.
"It's a dumb rule," the world number 13 said.
"Why are we making it not an individual sport? A huge part of tennis is as much mental as it is physical, and a big part of it is you need to be figuring it out on the court for yourself.
"I think it's ridiculous that you can be mentally not there, not good analytically, not good at kind of working through things and coming up with strategies, and you can have someone tell you what to do. I hate it."
Fritz will next play sixth seed Andrey Rublev after the Russian beat Italy's Fabio Fognini 6-7 (7-9) 7-6 (7-3) 6-2.
Greek fourth seed Stefanos Tsitsipas won 6-3 6-4 against Serbian Filip Krajinovic while Poland's Hubert Hurkacz lost to American John Isner 7-6 (7-5) 6-7 (7-5) 6-2.
Elsewhere, Canada's seventh seed Felix Auger-Aliassime defeated Australian Alex de Minaur 6-3 6-2 and compatriot Denis Shapovalov fought back to win 3-6 6-4 6-3 against American Tommy Paul.
Shapovalov will next take on world number one Daniil Medvedev next while Auger-Aliassime faces Italian Jannik Sinner.