Haywards Heath could be set for a winner-takes-all promotion play-off at Whitemans Green.
They lost control of the London One title race by going down 34-21 at London Nigerians.
Defeat, however, for third-placed Hertford leaves Heath needing just one point from their last two games to ensure at least second place, which would guarantee a play-off with the runner-up in South West One.
Heath could still go up as champions but they must beat Staines at home next Saturday and win at Thanet Wanderers on April 12 while overturning Southend's big advantage in the points difference column.
Southend currently stand on plus-342 compared to Heath's plus-277.
The leaders are at home next Saturday to a Hertford side who appear to have their minds on the Powergen Intermediate Cup. Southend's final game is at Norwich.
The National League dream is far from over for Heath.
Last season Basingstoke finished second and went up after a play-off win over Dings Crusaders who, along with Cinderford and Reading, are contesting the title this time around.
Basingstoke have gone on to do reasonably well in National Three South while Havant, who went up as champions, are likely to go straight back down again.
Tries from Nick Stocker and Nick Dinsdale gave Heath leads of 13-10 at the break and 21-17 early in the second half.
Owen Ashton kicked three penalties and a conversion.
Skipper Alex Meredith admitted his men were second best against a lively Nigerians side who scored five tries.
He said: "We let them come at us in the second half and ended up deserved losers.
"We are pretty much guaranteed a play-off place now and we just need to concentrate on getting great wins against Staines and Thanet.
"I'm obviously disappointed to lose but we will take it with good grace and put in a lot of hard work in training this week.
"How we respond from here will decide whether we go up."
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