London Lions were beaten 75-68 by Besiktas in the first leg of their EuroCup Women final in Istanbul.
Holly Winterburn scored a game-high 23 points for the Lions but the visitors trailed throughout the contest.
London Lions became the first British basketball team to reach a European final when they defeated Umana Reyer Venice in March.
They will seek to overturn their seven-point deficit in the second leg at London’s Copper Box Arena on 10 April.
Dana Evans led three-time Turkish league champions Besiktas’ charge with 20 points and Jovana Nogic added 18 for Aziz Akkaya’s hosts.
Karlie Samuelson (16) and Katsiaryna Snytsina (10) also recorded double figures for the Lions, who found themselves 18 points behind in the third period before 12 unanswered points kept their title hopes alive.
But Styliani Kaltsidou’s history-making team, who are also chasing a third straight domestic title in the Women’s British Basketball League, must hope they can mount a comeback in front of a home crowd in Stratford next Wednesday (19:00 BST).
The team with the higher aggregate score across the two legs will lift the trophy.