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.
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 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.
“I wasn’t able to sustain it for long enough. I played one or two good points, but then I was trying to finish the points a little bit early and shot selection goes down when you’re tired.”
Seventh seed De Minaur will now face Borna Coric in the quarter-finals after the Croat beat China’s Wu Di 6-3 6-3.
Also in China, fellow Briton Dan Evans lost 5-7 5-7 to Bulgaria’s Grigor Dimitrov in last 16 at the Chengdu Open.
Women’s world number one Ashleigh Barty is through to the semi-finals of the Wuhan Open after beating Croat Petra Martic 7-6 (8-6) 3-6 6-3.
The Australian, 23, will face Aryna Sabalenka for a place in the final after the Belarusian won 6-3 1-6 6-1 against Kazakhstan’s Elena Rybakina.
World number seven Petra Kvitova beat Ukraine’s Dayana Yastremska 6-2 6-4 to set up a semi-final clash with Alison Riske after the American upset world number three Elina Svitolina 6-1 6-3.