Britain fell behind in the Davis Cup final as Belgium's David Goffin came back from two sets down to beat Kyle Edmund in Ghent.
Goffin, the world number 16, won 3-6 1-6 6-2 6-1 6-0 against 20-year-old Edmund, who took a surprise two-set lead on his Davis Cup debut.
Andy Murray plays Ruben Bemelmans in Friday's second singles match at the Flanders Expo.
Britain are trying to win the team title for the first time since 1936.
The best-of-five tie takes place over three days, with Andy and Jamie Murray set to team up in the doubles on Saturday.
Around 1,300 travelling fans were in the arena on day one, amid tight security following the recent high state of alert in Belgian capital Brussels.