New-look Pompey couldn’t help prevent a familiar result as Hartlepool pulled off a 3-1 victory.
Guy Whittingham named a much-changed side, with seven alternations to the one which lost at Walsall last time out.
Yet despite an encouraging first half, the Blues found themselves behind to Jack Baldwin’s 18th-minute header following poor marking.
It had come against the run of play, with Pools keeper Scott Flinders making several fine stops to thwart the hosts, particularly against Darel Russell and David Connolly.
After the break, more disappointing defending allowed James Poole to increase the visitors’ lead in the 52nd minute.
Jed Wallace, making his first league start, did pull one back with a smart finish from the angle on 67 minutes.
But Whittingham’s men pushed forward for another, then in five minutes of time added on substitute Greg Rutherford sealed the scoreline.
It means the Blues’ winless run is stretched to 15 matches as League Two beckons.
And this was one of the lowest points of a desperate season as Pompey lost to the only team beneath them in the division - who had won just two matches all term going into the clash.
Shaun Cooper and James Keene were granted second debuts following their returns to Fratton Park, while Jed Wallace and Dan Butler got starts.
Also coming in was Adam Reed for his maiden Blues appearance, with Gabor Gyepes and Ricardo Rocha back in the team having missed that Banks’s Stadium defeat through injury.
In addition, Sam Sodje, Yassin Moutaouakil and Frankie Sutherland were on the bench following their recent arrivals.
When the match got under way, Pompey made a bright start with their newcomers at the heart of it.
Cooper fired in a vicious right-foot shot from 35-yards which had Flinders scrambling to beat out.
On 17 minutes, Butler’s free-kick from the right fell to Russell, who swivelled and crashed in a shot which drew a brilliant right-handed stop from Flinders.
The ball fell loose and Connolly produced a spectacular overhead kick at he far post which was somehow pushed around the post by Flinders’ left hand.
From the resulting corner, Russell headed wide at the far post when he should have done far better.
Yet in the 18th minute the deadlock was broken by Hartlepool, against the run of play.
Horwood delivered a free-kick from the right and Baldwin was there with a free header to nod past Simon Eastwood.
On 29 minutes Reed galloped down the right and pulled the ball back to Russell, whose powerful drive was superbly blocked by the flying body of Baldwin.
Connolly they had a snap-shot well saved by Flinders, while Simon Walton’s fierce effort at the other end was gathered by Eastwood at the second time of asking.
Pompey went in a goal down at the break but within seconds of the restart almost slipped further behind when Poole’s glancing header from Horwood’s cross flew just past the far post.
Keene had the home supporters on their feet in the 51st minute when a fierce left-foot shot from distance crashed against the stanchion.
But Hartlepool extended their lead in the 53rd minute when Poole beat the offside trap and produced a smart finish.
In the 63rd minute, Whittingham made a triple substitution with Ashley Harris, Moutaouakil and Sutherland coming on. Yet within seconds Eastwood had to pull off an excellent stop to stop Poole netting.
However, Wallace pulled on back in the 67th minute with a superb finish from the angle after Keene had caused problems in the air.
Pompey poured forwards and in the 85th minute Connolly shot from the angle which agonisingly trickled past the far post.
Five minutes time added on was given and, when Butler was caught in possession, Rutherford clinched a 3-1 win for the Pools.
POMPEY 1 (Wallace 67)
HARTLEPOOL UNITED 3 (Baldwin 18, Poole 52, Rutherford 90)
Pompey: Eastwood, Cooper (Harris 63), Reed (Sutherland 63), Connolly, Gyepes, Keene, Ertl (Moutaoukil 63), Russell, Rocha, Butler, Wallace. Subs: Sodje, Smith, Walker, Webster.
Hartlepool: Flinders, Austin, Humphreys, Collins, Horwood, Franks, Walton, Poole, Baldwin (James 74), Hartley, Wyke (Rutherford 90). Subs: McHugh, Monkhouse, Luscombe, Rowbotham, Holden.
Referee: Darren Sheldrake (Surrey).