A disappointing second-half display cost Bognor as they lost 4-2 at East Thurrock.
The defeat left the Rocks third in the final table and that gives them a home play-off semi-final on Wednesday night against Lowestoft.
It was the battle of the Rocks – with both teams sharing the same nickname. And it was the visitors who took the lead on seven minutes.
Ollie Pearce crossed the ball in and Ben Johnson fired home the opener. Back came the hosts and a thunderous effort from David Bryant crashed off the woodwork.
Bognor then extended their advantage in the 14th minute. They launched a superb move from the right, and Scott Chamberlain set up Pearce.
He had time to compose himself, before blasting home his low effort into the far corner.
In the 35th minute, the hosts pulled one back. Bryant’s well struck drive crashed in off the unfortunate James Crane.
The goal breathed new life into East Thurrock. They turned up the pressure but were unable to breach the Bognor defence.
In the second half East Thurrock capitalised on a poor display from Bognor.
The hosts struck an equaliser in the 55th minute. A cross from the right appeared to go in off Stuart Axten.
In the 70th minute the hosts took the lead when Mitchell Gilbey swept the ball home from eight yards.
And the scoring was completed 10 minutes from time. A strike from Sam Higgins took a wicked deflection to loop over Smith for the fourth goal.
The deserved victory for the hosts ensured they will avoid relegation from the Ryman premier, while Bognor finish in third place.
Bognor: Smith, Weir, Crane, Tuck, Axten, Robson, Chamberlain, Wills, Dodd, Pearce, Johnson.