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.
13. Horndean keeper Cameron Scott clears
Horndean keeper Cameron Scott clears. Picture by Nathan Lipsham Photo: Nathan Lipsham
14. A big smile from Horndean keeper Cameron Scott after the final whistle
A big smile from Horndean keeper Cameron Scott after the final whistle. Picture by Nathan Lipsham Photo: Nathan Lipsham
15. Portchester's Liam Robinson
Portchester's Liam Robinson. Picture by Nathan Lipsham Photo: Nathan Lipsham
16. Portchester's Liam Robinson, left
Portchester's Liam Robinson, left. Picture by Nathan Lipsham Photo: Nathan Lipsham