Bognor gave up top spot in the Ryman premier as they lost to a single goal at Havant.
Former Rocks loanee Wes Fogden got the only goal of a tight game to put Havant two points clear of Jamie Howell's team at the top with only one game to play.
It means Bognor must beat Met Police on Saturday and hope Havant do not win at Kingstonian.
The Rocks are now favourites to finish second for the second straight year - quite an achivement in itself - and go into a home semi-final play-off.
Bognor matched their hosts for most of the game but failed to work home keeper Ryan Young.
It was as big a game as you could wish for at Westleigh Park, with Bognor going into the tussle one point ahead of Havant at the top with just one game each to play after today's encounter.
That meant a win would clinch the title for Bognor but any other result would mean the race going to the final day, when Bognor host Met Police and Havant visit Kingstonian.
There was good news for the Rocks when the teams were announced, with Dan Beck fit enough after his recent hip problems to come back into midfield - a timely return with Gary Charman struggling after sustaining a foot injury in the win over Harlow on Saturday.
The massive nature of the game showed itself in news that kick-off had been put back to 3.15pm to allow many of the hundreds of people still queueing to get in a chance to see kick-off.
When it did get under way, Jimmy Wild enjoyed an early run down the left that was halted by a well-timed tackle by Dan Strugnell.
The first chance of note fell to Havant on seven minutes when Theo Lewis robbed Dylan Barnett and floated a cross into the box for Alfie Rutherford to head powerfully at goal but Dan Lincoln held it.
Soon Jimmy Muitt thought he had beaten his man down the right to get to the byline but it went out for a goal kick.
Havant were close again on 15 minutes when Rutherford returned the favour, crossing for Lewis, whose header was deflected over for a corner.
Dan Lincoln needed brief medical attention after falling heavily in a challenge with Jason Prior that earned him a fre-kick.
Wild won a free-kick in a promising position down the left but when Barnett swung it in, it flew off Harvey Whyte's knee and out for a goal kick. Then a long-range effort by Rory Williams was way over.
A foul on James Fraser brought a free-kick that Barnett chipped into the box and Sami El-Abd - still bandaged from his head injury at the weekend - beat keeper Ryan Young but there was no-one there to pick up the loose ball.
Then another rocks attack ended in Doug Tuck having two chances to shoot - one was blocked, the second was comfortable for the keeper.
A flowing Bognor move from their own area to Havant's came to nothing when Barnett's driven cross was just too far in front of Wild eight yards from goal.
Ed Harris' header looked destined for the bottom corner of Lincoln's net from a Brian Stock free-kick given harshly for a challenge on Rutherford by Barnett, but it was the wrong side of the post.
Rutherford was denied a goal against his former club by a superb block by Chad Field inside the area - and from the corner, Harris was unmarked but put his header straight at Lincoln.
Havant had a spell camped in the Bognor half at the end of the first half but in one break forward, a loose Havant pass almost allowed Wild to get in on Young - but it ran away from him.
Muitt had the first run of the second half, surging down the right then cutting inside but hitting a weak shot that was comfortable for Young.
Immediately at the other end Wes Fogden found space in a scramble in the box for a shot at goal but that too was all along the ground and a relatively-easy save for Lincoln.
Rutherford fired inches wide of the post when the Rocks only half-cleared a Strugnell cross in the 51st minute.
Lincoln had to stretch to punch away a high curling cross by Williams - but when Williams took the corner a quick exchanges of passes with Fogden put the former Rocks loanee into space in the area and he took one touch to wrongfoot a defender and a second to fire a fine shot across Lincoln and into the top corner.
Rutherford was then almost in on Lincoln as Havant tried to make the most of the moentum - then Whyte and Stock were booked for a clash on the halfway line.
Woodford was in the right place to clear off the line as Wild and Fraser tried to get on the end of a cross.
Bognor made their first change on 59 minutes, bringing on Ollie Pearce for Davies, with Whyte going to right-back.
The Rocks were indebted to Lincoln on 66 minutes for a point-blank save when Lewis' header met a Stock free-kick - then from the counter-attack that followed, Fraser ran at the full-back before curling a shot high and wide from 20 yards.
A second Rocks substitution saw James Crane replace Muitt. Crane slotted in at left-back with Barnett pushing forward.
Mike Carter replaced Lewis for the Hawks on 70 minutes.
Fogden was proving a thorn in the Rocks' side and set up Stock for a shot just outside the box but it was well over the bar.
Another Havant change meant the end of Rutherford's afternoon as Bradley Tarbuck replaced him.
Bognor were fighting to the end and Fraser's run through the centre ended with a looping shot that touched a defender and went behind for a corner, which was cleared after a brief scramble on the six-yard line.
Sub Tarbuck suddenly found himself with space to run at the Bognor defence but his shot was held at the second attempt by Lincoln. Then a Rocks attack ended with Crane volleying widly over.
Fogden was replaced by Ben Swallow with four minutes of the 90 left.
Beck was replaced by Darren Budd in the final minutes, one last attempt by the Rocks to save the day.
Tarbuck and Prior both had great opportunities to make it 2-0 on 90 minutes but one had a shot blocked and the other put his shot too close to the keeper.
Five minutes were added on to the surprise of most of the home fans. But Havant played the clock down well and the Rocks couldn't break through for a final chance to level the score and wrsetle back the initiative.
On to Saturday...
Havant: Young, Strugnell, Williams, Stock, Molyneaux, Harris, Fogden, Woodford, Prior, Lewis, Rutherford.
Rocks: Lincoln, Davies, Barnett, Tuck, El-Abd, Field, Fraser, Beck, Wild, Muitt, Whyte.
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