Pompey suffered their first pre-season defeat, despite Gareth Evans giving them the lead at Stevenage.
The winger steered home a neat finish midway through the first half, only for Ben Kennedy’s strike to ensure the scores were level at the break.
Susbtitute Andronicos Georgiou then added a brace to secure victory for the League Two outfit, despite a late consolation from Conor Chaplin.
Kenny Jackett had made extensive substitutions in the middle of the previous friendlies against Cork and the Hawks, but was more conservative with his changes at Broadhall Way.
Boro started brightly and almost took the lead when Joel Byrom’s free-kick was met by Kennedy, but Craig MacGillivray came to the rescue with a fine save.
Nathan Thompson then prevented Scott Cuthbert from converting the loose ball, while MacGillivray denied one of the hosts’ two first half trialists from the resulting corner.
But the Blues soon began to dominate and after Evans had sent one effort high and wide, Adam May’s 20-yard drive drew a decent stop from Paul Farman.
Then, following great work down the right from the lively Jamal Lowe, Brett Pitman had a shot blocked by Cuthbert.
Ronan Curtis was next to go close for the visitors, but he was unable to beat Farman after collecting a raking pass from Tom Naylor.
The deadlock was broken on 20 minutes, however, as Thompson slipped the ball through to EVANS, who neatly bent it into the net.
But Stevenage fought back and Kennedy was proving to be a constant menace with some probing runs forward.
He had already seen one attempt fizz agonisingly past the post before finding a leveller on 30 minutes.
The attacking midfielder was able to get away from Matt Clarke and then slam home a shot that left MacGillivray with no chance.
Pompey almost retook their lead, as Curtis ran along the edge of the box before curling in a shot that Farman did well to push away.
They kept plugging away as the interval approached, but neither Pitman or Lowe were able to make the most of promising openings.
Pompey emerged unchanged for the second half, while their opponents switched the entire 11 – with the introduction of five more trialists.
It was the visitors who were fastest out of the blocks, although a cleanly-struck Evans effort was comfortably kept out.
But Boro turned the contest on its head, as they took the lead in somewhat fortunate circumstances just before the hour mark.
Emmanuel Sonupe teed-up Georgiou and his shot took a wicked deflection off Naylor to wrong-foot MacGillivray and land in the net.
Jackett then made four substitutions, as Christian Burgess, Danny Rose, Chaplin and Oli Hawkins all entered the pitch.
The latter had a decent chance to draw Pompey level, but was unable to divert his header on target after meeting a pin-point cross from Lee Brown.
Stevenage, meanwhile, were attacking with pace and MacGillivray used his legs to keep out a Sonupe shot.
It was only a brief reprieve, however, as just seconds later, Sonupe’s delivery was emphatically finished by Georgiou at close range.
The game started to drift towards its conclusion after that, although Curtis was still proving a lively presence.
He did well to win back possession on the byline on 80 minutes, but could not force the ball home from an acute angle.
The summer signing also narrowly cleared the crossbar from long-range in the closing stages, as the Blues finished with a flourish.
But it was CHAPLIN who netted a consolation in the final seconds, controlling Curtis’ pass and slotting into the net.
Stevenage (4-1-2-1-2): Farman (Trialist 46); Vancooten (Fraser 46), Cuthbert (Trialist 46), Trialist (Wilkinson 46), Hunt (Trialist 46); Byrom (Trialist 46); Timlin (Sonupe 46), McKee (Trialist 46); Kennedy (Iontton 46); Trialist (Georgiou 46), Revell (c) (White 46)
Goals: Kennedy 30, Georgiou 57, 70
Subs not used: Martin, O’Donnell, Smith
Pompey (4-2-3-1): MacGillivray; Thompson, Whatmough (Burgess 62), Clarke, Brown; Naylor (Haunstrup 84), May (Rose 62); Evans (Chaplin 62), Lowe, Curtis; Pitman (c) (Hawkins 62)
Goals: Evans 20, Chaplin 89
Subs not used: McGee, Casey, Dennis, Smith