Withdean are en route to the Ryman League after beating AFC Wimbledon 2-0 in front of a 3,203 crowd.
Goals by Sam Francis and Roy Pook gave them victory in a bad-tempered clash of the promotion-chasers at Kingsmeadow which saw two players sent off and eight more booked.
The Sussex outfit are now ten points behind Wimbledon with nine games in hand after a 12th straight league win.
With leaders Wallingford having announced they will not go up at the end of the season, this was the biggest game of the season for both teams. If Withdean had lost it, plenty of fingers would have been pointed at referee Ian Mills.
The Oxfordshire official awarded the visitors an early penalty when Francis was hauled down by Glyn Shimell but then incredibly showed the Wimbledon goalkeeper only a yellow card.
Francis duly despatched the spot kick for his 37th goal of the season, but that was hardly the point.
With 72 minutes remaining, Wimbledon would have been staring down the barrel with only ten men and no substitute goalkeeper on the bench.
As it was, the home side were allowed to remain in the game until six minutes from the end when Pook drilled home Withdean's second after Shimell had failed to clear an Owen Hill cross following a quick break by James Bird.
What occurred in between was not pretty and hardly helped by a catalogue of decisions by the referee which bemused the crowd and frustrated the players of both teams.
Withdean's Shaun Grice was dismissed midway through the second half after a mass brawl which kicked off when Pook was floored in an off-the-ball incident, and Wimbledon defender Sean Daly was red carded 11 minutes from time following another clash off the ball, this time involving Withdean captain Damion Dobbyn.
What few chances there were fell to Withdean. Shimell made a flying save from an Ollie Rowland free-kick 25 yards out in the first couple of minutes and blocked a close-range effort by Pook after the interval.
At the other end, Jay Pickering had one-time Chelsea man Joe Sheerin in his pocket all afternoon, while good work by the Withdean defence ensured that Kevin Cooper never had a chance to add to his 30 goals for Wimbledon this season.
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