If GB win, first time since 1936 and it's the first time they've reached the final since 1978.
Matches are the best of 5 sets rather than 3 in the earlier rounds.
First match is in progress with Kyle Edmund in his Davis Cup debut leading David Goffin 2 sets to 1.
Second match is Andy Murray against Ruben Bemelmans later today.
Tomorrow's doubles match has Andy and Jaimie Murray against Kimmer Coppejans and Steve Darcis.
Sunday's singles are scheduled as Andy Murray against David Goffin and Kyle Edmund against Ruben Bemelmans though that may change.
GB will be favourites though Belgium has home advantage.