Emma Raducanu missed out on claiming her maiden WTA title as she fell to a 6-1 2-6 6-4 defeat by Clara Tauson in the WTA Chicago Challenger final.
She served to make it five games apiece in the third, only for Denmark's Tauson to secure a match-winning break.
Raducanu will now attempt to qualify for the US Open.
Tauson, also 18, powered to a 6-1 win in the opening set, but Raducanu - who dropped only one set on her way to reaching the final - regained her composure to lose only two games in the second, winning 75% of her first serve points.
Raducanu then went a break up in the nail-biting final set only for Tauson to recover and edge out her fellow teenager in an entertaining final.