East Grinstead are hoping to combine Sussex Trophy progress with a successful fight against relegation.
The London Three South East strugglers, finalists four years ago, beat Bognor 31-7 on Sunday to reach the last four.
They are joined by Haywards Heath, whose second team won 28-11 at Eastbourne.
Chichester were already in the last four and Worthing will join them if they win their delayed tie at Crowborough, conquerors of Horsham on Saturday.
East Grinstead were expected to face a tough time aginst a Bognor side chasing the London Four South East title.
Instead, they used their superior firepower out wide to overturn a 7-6 half-time deficit and win with a bit to spare.
Bognor's disappointment was completed when they lost key back Andy Shipp for the rest of the season with a broken collar bone.
Scrum half Stuart Sorrell was the star for Gs with a try, three penalties and a conversion.
Sorrell works in Formula One and will be called away soon for the start of the new season but Gs hope he stays around long enough to help them beat the drop.
Club spokesman Steve Perry said: "Stuart has been with the club for a while but he has come on a lot recently.
"We were delighted with the win. We played quite well in places."
Graham Bryce, called up just four hours before kick-off, scored Bognor's try with Shipp adding the conversion.
The Hampshire Avenue outfit are struggling with injuries but skipper Karl Flynn said: "I thought we had a good chance of winning at half time."
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