British number four Dan Evans lost 6-1 7-5 to sixth seed Jack Sock in the third round of the Citi Open in Washington.
The 26-year-old world number 82 defeated Benjamin Becker and Grigor Dimitrov in previous rounds but was soon 5-0 down to the world number 26.
Evans improved in the second set and had a set point on the American's serve at 5-4, but could not convert it.
He was then broken by Sock, who served out for victory in 76 minutes.
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