British number one Kyle Edmund suffered a 7-6 (7-4) 4-6 7-6 (7-5) second-round exit to 20-year-old world number 63 Frances Tiafoe at the Miami Open.
World number 26 Edmund lost the opening set tie-break but secured an early break to level at one set apiece.
Edmund broke again in the fifth game of the decider, but was broken back by the American as he served for the match.
Tiafoe surged clear in the tie-break and though Edmund saved five match points he lost in two hours 25 minutes.
Tiafoe won his first ATP Tour event at the Delray Beach Open last month, beating Germany's Peter Gojowczyk in the final to become the youngest American to win a title since Andy Roddick in 2002.