British number one Heather Watson lost in the Washington Open first round to wildcard Louisa Chirico.
It was the 23-year-old's first match since coming within two points of defeating eventual champion Serena Williams at Wimbledon a month ago.
But 19-year-old American Chirico, ranked 128, won easily, 6-3 6-0.
British men's number two Aljaz Bedene, 26, lost 6-7 (2-7) 6-4 6-1 to another wildcard, Dennis Novak, at the Generali Open in Kitzbuhel, Austria.
World number 57 Bedene reached the Hamburg Open quarter-finals last week but failed to recapture that form.
The Slovenian-born Briton suffered defeat in what was a first victory at ATP level for the Austrian world number 256 Novak.