Matt Tubbs leapt off the bench to become the late hero for Pompey as they snatched all three points with a 2-1 win at Luton Town.
The former Bournemouth man was among the substitutes as boss Paul Cook opted to go with Jayden Stockley as his lone striker.
And the move appeared to pay off when the Blues grabbed an eighth-minute lead through Gareth Evans.
The midfielder smashed home after a mix-up in the Luton box and Pompey looked in command at that stage.
But the home side were back on terms on 15 minutes as Craig Mackail-Smith levelled from the penalty spot after Josh McQuiod was felled in the box.
The game looked to be heading for a stalemate in the second half, even though Pompey kept pressing for a winner.
But neither goalkeeper had much to do with chances at a premium.
Their opportunity arrived as the clock ticked on to 90 minutes as fellow substitute Conor Chaplin burst through the inside left channel and saw his shot parried by the goalkeeper.
It left Tubbs with the simplest of tasks to slide into the open goal to secure all three points for the visitors.