Britain's Laura Robson lost in three sets to American Ellie Halbauer in the second round of the Granby National Bank Challenger tournament in Canada.
Having won her first match singles in almost two years in the opening round, the world number 913 lost 6-2 4-6 6-2 to the world number 498.
The 21-year-old returned from a wrist injury in June after 18 months out.
Meanwhile, British number two Aljaz Bedene lost 6-4 6-4 to Borna Coric in the second round of the Croatia Open.
Robson, who was 27 in the world in 2013, will play in the US Open next month thanks to a protected ranking.
She is able to do so as players are allowed to enter eight tournaments based on the ranking they had before a long-term injury lay off.