Pompey were left to rue poor defending as Felipe Morais’ double sent them spinning to a 2-1 defeat at Stevenage.
The Portuguese talent tormented Michael Appleton’s side and ended their five-match unbeaten run in League One.
Jon Harley got the Blues’ goal in a game they performed well in and created plenty of chances.
But their slackness at the back proved critical against the high fliers at the Lamex Stadium.
Pompey started much the more promising of the two sides.
Akos Buzsaky, Harley and Lee Williamson all had chances early on as Appleton’s side pushed.
But it was the home side who made the breakthrough after 14 minutes – and the Blues defence will not want to see the goal again.
Morais was afforded the freedom of the Pompey area to head home Lucas Akins’ cross with no blue shirt in attendance.
Morais was causing all kinds of problems in the hole as behind Marcus Haber for the remainder of the first-half.
But Pompey were not without their chances, and Izale McLeod’s cross shot was so nearly bundled home by Wes Thomas after 30 minutes.
Thomas then had a shot that was on its way until Mark Roberts stuck a knee out to put it behind for a corner.
Pompey then caused havoc in the home side’s box, but Williamson, Thomas and two efforts from Buzsaky couldn’t find the target.
Stevenage dangerman Morais was inches away from adding a spectacular second before the break with a 35-yard lob which sailed just over Mikkel Andersen’s bar.
The second half stared with Pompey firing on all cylinders and they looked to be level two minutes after the restart.
Thomas was the man who got clear and danced around the keeper, but Roberts somehow recovered to clear after the striker hit the ball towards the empty net.
But the Blues were level after 51 minutes, with Harley’s first Pompey goal.
The former Chelsea man did that in fine style as he picked the ball up 25 yards out and smashed a shot on the turn past Arnold.
Appleton’s side dominant and dictated the pace of the game after their leveller.
And Pompey fans were making their appreciation of their manager clear with chances aplenty lauding Appleton.
But the Blues boss would have been furious with the defending which gave Stevenage the advantage after 61 minutes.
Morais was the man who was again their tormentor as he received his short corner back from Luke Freeman.
The Portuguese ace whipped in a cross from the touchline which somehow evaded the Blues defence and went in at the back post.
And that was to prove the game’s key moment, with Pompey’s impetus draining and the home side able to hold on in relatively comfortable fashion.
The result leaves Pompey in 14th place in the table with 18 points from 14 games.