Naomi Osaka returned to action at the Cincinnati Open a week after retiring from her last match with a back injury.
But the four-time Grand Slam champion was unable to end a worrying slump in form, losing 6-4 7-5 to China's world number 44 Zhang Shuai.
Osaka, 24, has lost five of her past seven matches since being beaten by Iga Swiatek in the final of the Miami Open.
Japan's 2018 and 2020 US Open winner is 39th in the world, with the final Grand Slam of the year starting on 29 August.
Osaka's fall in the world rankings has followed a run that has seen her fail to make it past the second round in any of the five tournaments she has played since that run to the final in Miami in April.
She was beaten by 27th seed Amanda Anisimova in the first round of the French Open and decided not to play at Wimbledon due to an Achilles injury.
There was concern last week when she retired from her first-round match against Estonia's Kaia Kanepi at the Canadian Open, complaining of lower back pain.
"I felt my back from the start of the match and, despite trying my best to push through it, I just wasn't able to," said Osaka, who quit while trailing 7-6 (7-4) 3-0 to Kanepi.
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