Andy Murray is still searching for his first singles win since hip surgery after losing 7-6 7-5 to Tennys Sandgren in the Winston-Salem Open first round.
The former world number one played with conviction and fluency in the first set, but lost it on a tie-break 10-8.
Despite recovering from a double break down to square the second set at 5-5, Britain's Murray was broken again by American world number 73 Sandgren.
"I think there was some good stuff in there," Murray, 32, told BBC Sport.
"I think my ball striking was better than last week. I hit quite a few more winners, came to the net quite a lot.