British No 1 Kyle Edmund lost 6-3 6-2 against Chile's Cristian Garin in the first round of the Chengdu Open in China.
The world No 32 was broken twice in both sets by Garin, who is ranked just one place below in the world rankings, as he lost in 72 minutes.
Edmund has now lost four successive matches, after first-round defeats at the US Open and Cincinnati Masters last month.
Dan Evans began his campaign in Chengdu with a 6-3 4-6 7-6 (7-4) victory against wildcard Bai Yan, who was ranked 174 places below him in the world rankings.
Elsewhere, fellow Brit Cameron Norrie eased to a 6-1 6-4 victory against Peter Gojowczyk at the Zhuhai Championships in China.
The world No 68, who reached the semi-finals at the Atlanta Open in July, will now play third seed Gael Monfils in the second round.
Former world No 1 Andy Murray will continue his return to singles action at the inaugural tournament in Zhuhai against American Tennys Sandgren on Tuesday.
Meanwhile, Harriet Dart was knocked out in the first round of the Tashkent Open in Uzbekistan.
The 23-year-old, who is ranked world No 134, came through two rounds of qualifying to reach the main draw but was beaten 6-4 6-4 by Russia's Anna Kalinskaya.