Guernsey maintained their perfect home record with a 22-12 win over league leaders Chichester at Footes Lane in London One South.
Tom Ceillam got Guernsey's only try while debutant Luke Sayer kicked 17 points to seal a fine win for the island side.
"We knew we had to win this game, it doesn't get any bigger than this," boss Jordan Reynolds told BBC Guernsey.
"It's a pretty proud moment and we dug in deep today."
The win keeps Guernsey in fifth place but they are now just seven points off Chichester at the top.
"We knew we had to beat Chichester this week if we had any chance of having a shot by Christmas," added Reynolds.
"We wanted to be in contention by Christmas, this is just a stepping-stone but it's probably one of the biggest stepping-stones we're going to hit."