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."
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