Wycombe enjoyed the better of the opening 45 minutes and got the reward for their performance when Sam Wood turned home from eight yards.
However, Pompey rallied and levelled through Johnny Ertl's clinical finish from a corner, coming at the end of a strong spell of pressure from the former Premier League outfit.
The visitors, playing in their orange and yellow change strip, enjoyed a promising start and saw Danny Rowe’s left-footed shot deflected behind after a dangerous cross from Sam Wood wasn’t fully cleared.
At the other end, a couple of dangerous crosses threatened to trouble the defence but Alfie Mawson did well to divert Dan Butler’s delivery behind the goal with attackers lurking.
Jed Wallace and Sam Wood saw shots from distance miss the target while Peter Murphy wasn’t far away with a towering header from a Danny Rowe cross after a corner had been half-cleared.
Wanderers had to ride through some decent attacking spells from Pompey, spurred on by their vociferous home crowd, but as the half grew old, it was Wycombe who began to take control, and seemed keen to test keeper Paul Jones as Joe Jacobson, Alfie Mawson and Wood all applied pressure with shots from outside the box.
Wycombe’s best chance of the half came just before the interval, when a low drive from Jacobson was deflected and Paul Jones could only paw the ball to the side of the six-yard box, where Peter Murphy reacted quickest to get his shot away, but Paul Robinson was on hand to scramble it clear.
Craig Westcarr then fluffed an opportunity to make the breakthrough, mis-timing his connection with a low cross from the right and seeing the ball fly harmlessly over.
The signs were there that Pompey weren’t comfortable against the in-form Blues and they made a defensive change at the break, but it didn’t help them as they found themselves behind within five minutes of the restart.
Paul Hayes’ clipped cross was headed out to Peter Murphy, whose volley was met by an instinctive reaction by Sam Wood, and his shot deceived Paul Jones and found the back of the net.
It could have got better. Matt McClure was picked out in space inside the area but drilled his effort over the bar, when he perhaps even had time to take another touch before shooting.
The let-off seemed to wake Pompey from their post-interval slumber and they were almost celebrating an equaliser when James Dunne’s superbly hit half-volley rattled against the crossbar and bounced the safety.
Jed Wallace forced a good save from Matt Ingram at his near post as the midfielder bore down on goal from a tight angle, and from the resulting corner, Joe Devera missed a sitter, blasting over from close range after the delivery was flicked on.
Then Craig Westcarr fired over on the turn, collecting Alex Wynter’s low cross after the wing-back had burst into the box, before the inevitable equaliser arrived.
A corner from the right was met by a cleanly-hit strike from the half-time arrival Johnny Ertl, as he peeled off and found space to convert from around 12 yards, giving the unsighted Matt Ingram no chance.
Wycombe then regrouped well and regained a hold on the game that had deserted them for a 10-minute spell, and saw Jacobson’s free-kick deflected wide after Matt McClure was bundled over outside the box.
However, Pompey could sense a winner and weren’t far away when Jed Wallace’s low shot was heading for the bottom corner before Matt Ingram stuck out a big hand to keep the ball out.
Blues looked content to settle for a point, having given their all to force their way into a winning position earlier in the game, and held on resolutely to maintain their unbeaten start to the season away from home. They even had half-chances to snatch it at the death, when Cowan-Hall fired straight at Jones and Sam Wood volleyed over, both from the edge of the box.
Wycombe: Ingram, Jombati, Jacobson, Mawson, Pierre, Scowen, Murphy, Rowe, Wood, McClure (Cowan-Hall 77), Hayes (Kretzschmar 87).
Subs: Richardson, Bloomfield, Lewis, Walker, Holloway.
Portsmouth: Jones, Robinson, Devera, Whatmough (Ertl 45), Butler, Wynter, Dunne, Wallace, Hollands, Westcarr (Barcham 86), Agyemang (Storey 61).
Subs: Poke, Webster, Shorey, Close.
Ref: Mr G Sutton.
Attendance: 16,171 (729 from Wycombe)