Pompey were looking to build a big gap between the sides immediately below them in the League One table.
But instead a 2-1 defeat at Charlton saw the hosts move to within two points of their fourth-placed opponents.
Ronan Curtis cancelled out Joe Aribo’s opener just before the break, only for Lyle Taylor to grab what proved to be the winner early in the second half.
Kenny Jackett believes the Addicks’ midfield display proved the difference, but has not ruled out a late flourish to the season.
The Blues boss said: “The winning goal was scored when we were down to 10 men because Gareth Evans went into the boards and had to go off for a bit.
“There were some key moments after that – Jamal had one chance following a very good ball from Brett. We did everything we could to get back into the game.
"With 10 games to go, we might need to win eight or nine of them, but you have to believe that you can do that."
“It is those fine margins that define football matches and they just didn’t go our way on this occasion.
“Charlton were excellent in midfield and their better players are in that area – they played to their strengths.
“It’s not necessarily the formation, but the quality of their players and it’s something we need to learn from if we do play them again.
“With 10 games to go, we might need to win eight or nine of them, but you have to believe that you can do that.
“If we can build some momentum – which I do think we’re capable of – then we can put pressure on the top two.”
Brett Pitman came off the bench and looked bright playing just behind fellow substitute James Vaughan.
Jackett said: “He came that little bit deeper and was more in the role of a provider rather than a scorer.
“I did think that he gave us something and there was one great reverse pass that led to our best chance of the game.
“It was a good little cameo and a really positive performance from a substitute. He showed plenty of hunger.”