They travelled to Woking Park to face promotion-chasing Westfield on Saturday afternoon.
The scoreboard was not troubled until the 74th minute, when second half substitute Sam Murray scored for the second game in succession.
Home goalkeeper Sam Wheeler had what looked like a routine clearance to make, but he let the ball slip under his foot and Murray pounced, drove into the penalty area and slotted home before a sliding defender could make an interception.
Westfield took little time in responding.
Just two minutes had passed when Manolis Gogonas let fly from just outside the area, with his effort curling past Addison’s dive and inside the post.
Mayowa Adaje then hit another long-ranger on 80 minutes that flew in for Westfield’s winner.
WESTFIELD: 1 Sonny WHEELER; 2 Zack CHISLETT: 3 Billy NUTBEAM; 4 Alfie HUCKLE; 5 Caleb WRIGHT (15 Lewis JACKSON, 81′); 6 Niran BUTLER: 7 Jamarie BRISSET (16 Mayowa ADAJE, 70′); 8 Andrew CROSSLEY; 9 Manolis GOGNAS; 10 Jordan ALVES (14 Theo WHITE, 69′); 11 Archy TAYLOR. Not used: 12 Quincy ROWE; 17 Samuel BALOGUN.
Booked: Huckle, Crossley.
GUERNSEY: 1 Josh ADDISON; 2 Niall HAINSWORTH; 3 Dave MERRIS; 4 Tom DODDS; 5 Jacques CAUVIN; 6 Seb VANCE; 7 Charlton GAUVAIN; 8 Matt LOARING (C); 9 Owen WALLBRIDGE; 10 Dave RIHOY; 11 Brandon WALLACE (14 Sam MURRAY 67′). Not used: 12 Alex WARD.
Booked: Loaring.