The Hawks slipped to a 2-1 defeat against fellow Vanarama Conference South play-off rivals Whitehawk at Westleigh Park.
JJ Hooper gave the home side a first-half lead against the run of play with a close-range finish.
But the visitors hit back with two goals in the final 13 minutes through Jake Robinson and Ahmed Abdullah.
Lee Bradbury’s side now meet Boreham Wood in a two-legged semi-final, with the first leg at home on Wednesday night.
Gosport Borough signed off their Vanarama Conference South campaign in style as they earned an impressive 2-1 victory over champions Bromley to seal a sixth-place finish – their highest-ever league position.
Matt Paterson and Adam Wilde were on target for Alex Pike’s men as they completed an impressive double over the league’s top side.
Anthony Cook grabbed a consolation for the visitors in stoppage-time.
Bognor’s Ryman League premier division campaign came to a disappointing end as the Rocks lost to a goal scored by Nyewood Lane old-boy Kane Wills in their 1-0 defeat to Margate.
Wessex League premier division champions Petersfield ensured they maintained an unbeaten league run stretching back to mid-October with a final-day 1-0 win at Newport IoW.
Will Essai was the goalscorer for Ian Saunders’ men who ended the campaign with a magnificient 96-point haul from 40 games.
Elsewhere, in-form AFC Portchester claimed third spot in the Wessex League premier division with a 3-1 win over Fawley.
Rob Evans, Nikolai Littleton and Joe Noakes scored for the Royals.