After losing at home to lofty Shaftesbury and away to lowly Hythe, the Deans were beaten 2-1 at title favourites Hamworthy.
Lee Francis opened the scoring after an error from visiting keeper Cameron Scott - punching a cross instead of catching it.
Cameron Munn doubled the advantage after half-time before Brandon Miller lashed in a late consolation free-kick after Harry Jackson had his shirt pulled on the edge of the 18-yard box.
After being in the top two for most of the season, Horndean are now down to fifth. Their title hopes are virtually over - Hamworthy are 10 points ahead with two games in hand - and even the play-off place, in second, is now a tall order.
Even if Horndean win their last four games, Shaftesbury would require just eight points from six games to finish ahead of them.
‘We did ok, that was the bottom line,’ said Deans No 2 Jason Mann of the loss to the FA Vase semi-finalists.
Mann was again standing in for manager Michael Birmingham, who was serving the second of a three-game stadium ban for a red card against Blackfield recently.
‘We set up to try and nullify them, to frustrate them, with five at the back. That was working well up to half-time when the keeper (Scott) punched a ball he should have caught.
‘The disappointing thing was we didn’t look like we had the belief after that to get back in it.
‘The players were all sevens out of 10, which against a team like Hamworthy is not enough.’
Mann was forced into changes with centre half Luke Dempsey suspended and captain Ash Howes injured. In came ex-Baffins pair Miller and Sam Emeney.
‘We’re not going to let the season peter out,’ declared Harris. ‘We’re still fighting for that second spot but we’re going to need to win our last four games and hope others drop points.’
Hamworthy’s win against Horndean maintained their five-point advantage of Shaftesbury, with both teams having six games left. The stage is therefore set for an epic encounter on Tuesday when they top two meet in Poole.