British world number three Andy Murray suffered a surprise second-round defeat to unseeded Russian Teymuraz Gabashvili in the Washington Open.
Murray, 28, had chosen to play an extra tournament in the run up to this year's US Open, which starts in New York on 31 August.
But after a bye to the second round, the top seed lost 6-4 4-6 7-6 (7-4) to the 30-year-old world number 53.
"I was exhausted at the end because my body was really tired," said the Scot.
He will now play Masters events in Montreal and then Cincinnati in the next two weeks in preparation for Flushing Meadows.