Marcus Harness gave the Blues the lead against his old club but Colby Bishop levelled from the penalty spot to leave the half-time scoreline 1-1 between Town and Portsmouth at Portman Road.
Wes Burns and Tyreece John-Jules returned to the line-up for Kayden Jackson and Freddie Ladapo, who dropped to the bench.
Burns, who missed the trip to Plymouth having been away on international duty with Wales, came in wide on the right, while John-Jules was the lone central striker with ex-Pompey pair Harness and Conor Chaplin behind him.
For Portsmouth, Ipswich-born Joe Morrell came into the midfield for Tom Lowery, who has had a hamstring injury, while former Town loanee Ryan Tunnicliffe was on the bench and on-loan Blues frontman Joe Pigott was ineligible.
Town started the game positively, controlling the game with Portsmouth unable to get on the ball.
The Blues created their first chance in the eighth minute, Burns cutting back from the right to Chaplin but the former Pompey man scuffed his shot.
Within a minute, Harness sent Leif Davis away on the left and his cross found Burns deep on the right but his shot was blocked.
Following the corner, the Blues went close again, Davis cutting back from the left and John-Jules trying to turn the ball goalwards from behind him. It looked to be going wide before hitting a couple of defenders and going behind.
From that flag-kick Town again almost went in front, Chaplin hitting a shot on the turn against the post after a loose ball had fallen to him.
The Blues maintained the pressure and in the 11th minute Dane Scarlett was shown the game’s first yellow card for a late challenge on George Edmundson midway inside the Portsmouth half to the right.
In the wake of Lee Evans’s free-kick, Chaplin crossed from the left and Edmundson’s looping header was claimed under the bar by visitors keeper Josh Griffiths.
Portsmouth saw a bit more of the ball in Town’s half as the game approached the 20-minute mark but Scarlett’s hopeful effort from distance was miles away from Christian Walton’s goal.
In the 22nd minute, a Pompey free-kick from the right was punched to the edge of the area by Walton from where Josh Koroma hit a shot against Janoi Donacien, then a subsequent effort was blazed well over.
Town attacked from the free-kick, John-Jules seeing a shot blocked, then the ball looped out to the left of the box from where Davis hit an effort which deflected over Griffiths but onto the roof of the net. From the corner, Edmundson headed over.
The Blues had had more than a few chances and in the 25th minute they made one count, and perhaps inevitably both Town’s former Pompey players were involved.
John-Jules tussled with Morrell on halfway and the ball fell to Chaplin, who brought it forward before playing in Harness, who confidently slipped his sixth goal of the season past the advancing Griffiths and into the net before celebrating in front of the Sir Alf Ramsey Stand and not too far away from his old fans.
But six minutes after the Blues went in front, Portsmouth were awarded a penalty. Luke Woolfenden allowed Scarlett to get behind him as the ball was played into the right of the box and Walton upended the on-loan Tottenham forward. Referee Charles Breakspear pointed straight to the spot and yellow-carded the Town keeper.
Colby Bishop took the penalty and hit it low to Walton’s right and the ball just made it under the keeper who will have been frustrated not to have saved it.
Town went about restoring their lead and in the 36th minute skipper Sam Morsy curled a shot over from the edge of the box.
Two minutes later, Burns took down Evans’s deftly-weighted ball over the top and John-Jules played it into the path of Chaplin, whose effort was scraped wide.
On 44, Davis played a pass from deep on the left to Chaplin in the centre circle where the forward took it down superbly before playing an equally impressive ball into the path of Burns on the right. The Welshman took it on to the edge of the area before hitting a shot which Griffiths batted away. From the corner, Harness’s stooped header was easy for the keeper.
Just before half-time, Chaplin looked to be tripped in the area but remained on his feet and despite Town protests, referee Breakspear wasn’t interested.
Town will feel they had done enough to go in ahead at the break, Chaplin and Harness having done brilliantly to create the opening goal and the Blues having created plenty of other opportunities.
But as so often over the last couple of weeks, Town were made to rue a defensive error which allowed Pompey back into the match.
Town had been more subdued immediately after conceding but had begun to look more threatening close to half-time.
Town: Walton, Donacien, Woolfenden, Edmundson, Burns, Evans, Morsy (c), Davis, Chaplin, Harness, John-Jules. Subs: Hladky, Keogh, Vincent-Young, Ball, Edwards, Jackson, Ladapo.
Portsmouth: Griffiths, Robertson (c), Ogilvie, Pack, Bishop, Dale, Morrell, Scarlett, Raggett, Koroma, Morrison. Subs: Oluwayemi, Swanson, Tunnicliffe, Curtis, Hackett, Jacobs, Mingi. Referee: Charles Breakspear (Surrey).