Britain's Andy Murray said he played "only top-70, top-60 level" tennis as he lost to Alex de Minaur in the last 16 of the Zhuhai Championships.
Former world number one Murray - now ranked 413 after returning from hip surgery - won the first set against the Australian in China.
But the 32-year-old slipped to a 4-6 6-2 6-4 defeat by the world number 31.
"Physically, I was struggling," he said. "My level dropped off a little bit and he capitalised on that."
Murray: It's naive to think I can return to my best
Murray won two matches on the lower-level Challenger Tour at the Rafa Nadal Open last month.
But Tuesday's three-set win over Tennys Sandgren in the first round of the Zhuhai Championships was his first singles victory on the ATP Tour since January.
"My body held up well after two pretty long matches but the rest of my body is just tired," Murray said.
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