Guernsey moved out of the National Three London relegation places after a 36-25 win at London Irish Wild Geese.
Luke Sayer scored twice as Guernsey held their opponents 12-12 at half-time before tries from Dave Davison, Tom Ceillam and Matt Hema sealed the win.
"It was probably one of the best performances a Guernsey team has displayed in many years," head coach Jordan Reynolds told BBC Guernsey.
Victory over the high-flying Wild Geese lifts Guernsey to 10th.
The bottom three teams in the competition will be relegated and Guernsey host fellow strugglers CS Rugby and Tring in their next two matches.
|Bottom of National Three London & South East|
|12||Amersham & Chiltern||34|
|14||CS Rugby 1863||30|
"The fact is that this is just one performance, we've got to push that on through to CS Rugby," Reynolds added.
"It's an achievable win so we've got to have the right mindset going into it."
Reynolds was full of praise for the way his side inflicted just a fourth home defeat of the season on their opponents, who dropped to third in the league as a result of the loss.
"We did a fair bit of study on Irish and knew we had to nullify a few areas of theirs and we did that exceptionally well," said Reynolds.
"While Irish were trying to work out how to get to Plan B, we suffocated them of the ball and started playing some good rugby.
"We've got five games remaining and we'll target every one of those as wins from hereon in.
"We've got good confidence in the side. We are out of the relegation zone, but we've been up and down all year, so we have to be consistent over the next coupe, of weeks."