British number two Dan Evans lost in the Washington Open first round after appearing to struggle in hot conditions against Japan’s Yoshihito Nishioka.
Evans, 29, lost nine of the final 10 games in a 6-4 6-1 defeat.
World number 51 Evans started the match strongly by breaking Nishioka’s serve for a 2-0 lead, but could not back it up by holding in the next game.
Nishioka, ranked 77th, broke again for 5-4 and served out the opener, going on to dominate a 27-minute second set.
British number Kyle Edmund returns to action for the first time since Wimbledon when he plays 22-year-old South African Lloyd Harris, who took a set off Roger Federer in the first round at the All England Club, later on Tuesday.