Murray was first to put points on the scoreboard after a 19-ball opening rally but it was the 23-year-old Swede who broke the Scotsman’s serve and claimed the first game.
It was the beginning of a toe-to-toe encounter that saw Murray fail to convert four set points as world number 115 prevailed in a gruelling 80-minute first set.
Murray will now prepare for next week’s National Bank Open Presented By Rogers in Montreal, which is a Masters 1000 Series event.
There were British wins, however, for Kyle Edmund and Jack Draper at the Citi Open. Edmund, who is returning from injury on the main tour, defeated Japan’s Yosuke Watanuki 6-4 7-6, while Draper progressed after defeating American Stefan Kozlov 7-5 6-2.
Elsewhere, Emma Raducanu’s professional doubles debut ended in straight sets as the Brit and fellow teenager Clara Tauson fell 6-4 6-1 to Romanian Monica Niculescu and the Czech Republic’s Lucie Hradecka in the opening round.