Serb Nikola Cacic beat British number three, Aljaz Bedene, who, in the first round of the Croatia Open, ranked 692 places below him.
Cacic, who ranked 761 in the world, on Tuesday, came back from a set down to win 5-7 6-4 6-4.
In Washington, in the first round of the Citi Open, Dan Evans, British number four, won 6-2 6-1 against Benjamin Becker, the German world number 96. Evans needed 49 minutes to win.
Evans will now be facing Bulgaria’s Grigor Dimitrov.