Gareth Evans found the target as a dominant Pompey won at Walsall to keep alive their play-off hopes.
The Blues took charge following a scrappy first half, although it seemed as though they would not be able to find a way past the struggling Saddlers.
But their patience was rewarded 11 minutes of time when Evans stroked home to seal a third successive victory.
Kenny Jackett made one change from the side that beat Oxford on home soil the previous week.
Evans returned to the starting line-up in place of Connor Ronan, while the fit-again pair of Stuart O’Keefe and Oli Hawkins were both named on the bench.
There was a slight reshuffle, however, as Nathan Thompson switched to right-back, with Anton Walkes handed the holding midfield role.
With every other League One match played the previous day, there was no-one else to take the attention away from Pompey and Walsall.
Unfortunately, the first half failed to make the most of the spotlight, with no clear-cut opportunities arriving at either end of the pitch.
There were half-chances peppered throughout, though, with most of them falling the way of the visitors.
Jamal Lowe saw an early effort blocked, while Dion Donohue and Luke McGee both acted quickly to beat opposition players to the ball.
Brett Pitman did well to get a shot away after a Naismith flick, although there was not enough pace behind it to trouble Liam Roberts in the Saddlers goal.
Arguably the best Blues chance arrived on 12 minutes when Ben Close made a clean connection with Evans’ delivery, only to send the ball straight at Roberts.
A couple more Pompey chances went off target as the half progressed, as Jack Whatmough was unable to make the most of space afforded to him at the far post.
Pitman then met another Evans cross with a deft header – although it was slightly too deft and the ball flashed past the post.
Close also fired into the side netting, while Kieron Morris dragged wide in a rare opening at the other end.
But the biggest reaction from the stands came when Nicky Devlin was booked for a foul on Naismith, with some irate home supporters believing it was a soft decision.
It was Pompey who looked the more attack-minded side once the second half resumed and they launched several quick forays forward.
One of them led to a chance for Donohue, whose thunderous effort from the left side of the box narrowly cleared the crossbar.
Naismith – still getting booed for having the temerity to be fouled earlier – was next to go close and his shot took a slight deflection to divert the ball wide.
It was the visitors on top and Roberts had to grab Pitman’s looping header underneath his own bar after the skipper had met Close’s cross.
The lively Naismith was then afforded space to surge forward on 65 minutes and drive the ball just wide.
He then turned provider, sending in a teasing low cross that a lunging Pitman was agonisingly unable to reach at the far post.
But Walsall appeared to have weathered the storm and soon had their own chance to break the deadlock.
Luke Leahy sent in a teasing delivery from wide on the left and Amadou Bakayoko came charging in to meet it, but could not keep his shot down.
But the Blues almost scored themselves when Jamal Lowe was brought down and Pitman’s free-kick had to be turned over by Roberts.
A goal did arrive on 79 minutes, however, as the visitors finally made their second half dominance count.
Lowe laid the ball into the path of EVANS on the edge of the box and he hit a sweet first-time shot past Roberts’ grasp and into the net.
Walsall tried to find a way back into the contest, but the Pompey saw out the closing stages with relative ease to ensure a fine opening to their Easter double-bill.
Walsall (4-2-3-1): Roberts; Devlin, Fitzwater, Guthrie, Leahy; Cuvelier (Dobson 72), Chambers (c); Morris, Oztumer (Shaibu 84), Ngoy (Kouhyar 67); Bakayoko
Booked: Devlin, Bakayoko, Guthrie, Morris
Subs not used: Gillespie, Roberts, Kinsella, Flanagan
Pompey (4-1-4-1): McGee; Thompson, Whatmough, Clarke, Donohue; Walkes; Lowe, Evans, Close (Ronan 77), Naismith; Pitman (c)
Goals: Evans 79
Booked: Donohue, Walkes
Subs not used: Bass, Burgess, Haunstrup, O’Keefe, Kennedy, Hawkins
Referee: Darren Handley