Britain's Katie Boulter faded after a fast start as she lost in straight sets to Belgian 16th seed Elise Mertens in the Miami Open second round.
Mertens saw off the break point to hold for 3-1 and seized control - forcing errors as she troubled Boulter with variety of play - winning 6-4 6-1.
Dan Evans and Cameron Norrie play in the men's event later on Friday.
British number one Evans, seeded 19th, plays American Frances Tiafoe after 23:00 GMT, while Norrie takes on Bulgarian ninth seed Grigor Dimitrov in the day session.