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.
1. Harrison Brook about to put Portchester ahead from the penalty spot
Harrison Brook about to put Portchester ahead from the penalty spot. Picture by Nathan Lipsham Photo: Nathan Lipsham
2. Harrison Brook has just put the Royals ahead from the penalty spot
Harrison Brook has just put the Royals ahead from the penalty spot. Picture by Nathan Lipsham Photo: Nathan Lipsham
3. Portchester's Harrison Brook in possession
Portchester's Harrison Brook in possession. Picture by Nathan Lipsham Photo: -
4. Portchester v Horndean
Portchester v Horndean. Picture by Nathan Lipsham Photo: Nathan Lipsham