Leaders AFC Portchester could have opened up a 14-point lead over third-placed Horndean, having played four games more, with victory in their final On-Site Stadium fixture of 2022/23.
But the Deans recovered from going behind early on to a Harrison Brook penalty to claim a superb 2-1 success.
Rudi Blankson levelled within minutes and top scorer Zack Willett plundered the decider - his 40th Wessex Premier goal of the campaign.
Horndean are now eight points behind the Royals with the first of those four matches in hand coming at home to fourth-placed Bemerton on Tuesday.
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