Barry Corr's late strike gave Southend a narrow victory over 10-man play-off chasers Stevenage.
The visitors faced an uphill struggle after Peter Winn was dismissed for a challenge on Sean Clohessy just before the half-time break.
Boro were given a lifeline when Bilel Mohsni fouled Byron Harrison in the box but John Mousinho saw his resulting penalty saved by Glenn Morris.
And Corr latched onto Ryan Hall's cross and won with with a 12-yard drive.
Stevenage manager Graham Westley told BBC Three Counties Radio:
"Everyone that watched it would know the lads put everything into the game, they had some difficult decisions to contend with and a disappointing outcome.
"It wasn't a disappointing performance. The work ethic and organisation and fight of the players deserved a lot more than they got.
"We knew that if we won all four games (remaining), we had a chance of automatic promotion. I think automatic has probably gone out of the window now."