It represented a first for Pompey at Fratton Park this season.
Paul Cook’s men were handed an Oxford United lesson to suffer a painful blow to their automatic promotion aspirations.
The ball was almost like a hot potato to us in the first halfGareth Evans
Yet it was the manner in which Michael Appleton’s fearless side pulled off their triumph which raised concerns.
The Blues were hassled, harried and knocked out of their usual flowing stride during a dominant opening 45 minutes from the U’s on Saturday.
Despite hosting three Championship clubs this season, no side has managed to stand up so successfully to a Pompey team previously unbeaten at home since September 29.
It was a marked improvement from the hosts after the break, ironically when Jordan Bowery climbed off the bench to net the decisive 76th minute goal.
And for Gareth Evans, Pompey need to learn from Oxford’s successful game plan.
He said: ‘It was one of those days you want to forget about and we have to learn from that game plan and take it into future games.
‘Fair play to Oxford to come here and play the way they did. I think a lot of teams have been a bit scared of the way we play so are scared to come out and take us on.
‘Oxford, though, came out of the traps and really got into our faces and it’s not something we have really experienced at Fratton.
‘A lot of teams have come and parked the bus and it was a different challenge, they got in our faces and tried to dispossess us quite a lot so fair play to them.
‘The ball was almost like a hot potato to us in the first half.
‘Once we came out in the second half I felt we were much the better team for 20-25 minutes, we were playing in their half.
‘As we kept ourselves in the game until half-time, I felt we were going to come out and really dominate after the break and we did, their goal was against the run of play.
‘In the second half we were far the better team and I think that was the case at their place as well.
‘Overall it was a tight game, not a whole lot of chances created either end and almost a bit of a stalemate.’
The Blues remain in fifth place following only their fourth defeat of the League Two campaign.
Crucially, they are now seven points adrift of Oxford, who presently occupy the third of the promotion places.
Even further ahead are Northampton, the leaders boasting a daunting 14-point advantage over Cook’s men.
This weekend attention is fixed on the FA Cup visit of Bournemouth, but the next league match at Morecambe on February 2 remains an important fixture.
Evans added: ‘There are loads of points to play for. We have 20 games left, teams have got to come here, teams have got to play each other, so you can find yourself back in the mix in no time.
‘We have to go into the Morecambe game and get three points then everything starts to look rosy again.
‘There are a lot of teams right in the mix and we have to stick together as a club.’