Naismith strike not enough for victory.
Pompey missed a chance to climb into the automatic promotion places after conceding a late equaliser against Morecambe.
It was the hosts showing all the ambition, although their dominance was receiving no reward as the Shrimps held firm.
But Kal Naismith came off the bench to break the deadlock after the interval and the points looked to have been secured.
Lee Molyneux spoiled the party, however, with a leveller to stop the Blues overhauling defeated Carlisle.
Paul Cook unsurprisingly stuck with the same side that beat their promotion rivals at the weekend.
And they immediately set about trying to attack their opponents, who were prepared to sit back and soak up the pressure.
The Blues were passing the ball around with style and were only let down by some wayward finishing.
But Morecambe were also indebted to their goalkeeper Barry Roche, who tipped over a Carl Baker shot before using his feet to deny Gary Roberts.
Roberts was dictating play in the middle of the park and played an exquisite pass through to Eoin Doyle, only for Alex Whitmore to come across and make a fine challenge.
There was not too much action at the other end of the pitch, although Christian Burgess headed off the line before referee Stephen Martin finally spotted his assistant flagging for a foul on David Forde.
It was soon the Blues back on the attack and when another fine Roberts pass released Gareth Evans, it took another Roche save to keep the scores level.
Kyle Bennett then sent a couple of long-range efforts narrowly off target, while Eoin Doyle headed over from a Baker cross.
But Morecambe almost forged ahead completely against the run of play on 32 minutes, with Forde doing brilliantly to stop Aaron Wildig from converting Paul Mullin’s cut-back.
Normality resumed for the rest of the half, but despite Pompey’s dominance, they couldn’t find a way through.
Paul Merson was introduced to the crowd at half-time ahead of his induction into the club’s Hall of Fame.
And Roberts was certainly trying to match the former Blues star in the creativity stakes, with a low cross just about grabbed by Roche before it could reach Eoin Doyle.
But the second half was lacking the excitement of the first and Cook tried to liven things up with a substitution on 56 minutes, replacing Baker with Conor Chaplin.
Bennett and Roberts combined well as the hosts tried to find a breakthrough, although the latter’s ball was not quite right for Eoin Doyle.
Cook’s second switch saw Naismith replace Bennett on the left – and it paid instant dividends.
NAISMITH controlled Michael Doyle’s pass with his first touch and then, with his second, steered the ball into the bottom corner from 25 yards.
The Blues continued to attack and Chaplin sent a snap-shot wide before Roberts saw his effort saved after being teed-up by Naismith.
But the game was drifting towards a conclusion, until Morecambe suddenly found themselves level on 83 minutes when the ball was worked across to Molyneux, who drove home a precise finish.
Pompey tried to hit back and the award of six additional minutes provided some hope that a winner could arrive.
But the Shrimps somehow survived a corner that ricocheted around their box to depart with a share of the spoils.
Pompey (4-2-3-1): Forde; Evans, Burgess, Clarke, Stevens; Linganzi (Rose 79), M.Doyle (c); Baker (Chaplin 56), Roberts, Bennett (Naismith 63); E.Doyle
Goals: Naismith 64
Subs not used: O’Brien, Whatmough, Lowe, Hunt
Morecambe (4-2-3-1): Roche; Wakefield (McGowan 72), Edwards, Whitmore, Molyneux; Fleming (Murphy 79), Rose (c); Turner (Conlan 55), Wildig, Ellison; Mullin
Goals: Molyneux 83
Subs not used: Nizic, Duckworth, Evans, Jordan
Referee: Stephen Martin
Attendance: 15,592 (71 away fans)