Kusini Yengi came off the bench to score a dramatic stoppage-time equaliser for Pompey against Bristol Rovers.
Luke Thomas netted midway through the first half for the Gas and that looked to be enough to earn them a first win at Fratton Park since 1975.
But debutant Yengi converted a Joe Rafferty cross at the death to ensure that the Blues earned an opening day point.
John Mousinho handed competitive debuts to four of his summer signings – while there were another six on the bench.
Will Norris, Regan Poole, Anthony Scully and Gavin Whyte were all given an opportunity to impress from the start.
And it was a proud moment for local boy Marlon Pack, who led the team out after being named skipper.
There was a familiar face in the opposition line-up, with John Marquis chosen to lead the line for the Pirates.
The heavens opened just before kick-off and the rain only got heavier during the early stages of the first half.
It was the visitors who seemed to adapt quicker and Marquis fired an early effort over, while Aaron Collins could only fire into the side netting.
There was guidance received ahead of the season that referees would be clamping down on certain indiscretions – and both sides racked up three bookings each before the break.
Pompey came close to breaking the deadlock on 12 minutes, when Rafferty found Scully at the back post.
He laid the ball off for Whyte just a few yards out, but the winger’s effort was deflected over the crossbar.
It was Rovers who broke the deadlock, however, as Jevani Brown collected Sam Finley’s pass and found Thomas in acres of space to slam home.
The hosts improved after that and looked the more dominant side for the remainder of the opening half.
A series of corners suggested a breakthrough might be near and when Pack found Whyte in the box, he fired inches past the post.
Norris’ long ball up the pitch was then nodded down by Colby Bishop, with Tom Lowery just beaten to the ball by keeper Matt Cox.
Lowery then tried his luck from the edge of the box, but was unable to generate enough power and Cox gathered low to his left.
There was another opening when Joe Morrell capitalised on a loose ball and was crudely brought down by James Wilson, with Pack unable to keep his free-kick down.
Eight minutes added on was another result of new regulations, designed to clamp down on teams wasting time.
And the Blues might have gone in level at the interval, with Lowery’s delivery ricocheting off Bishop’s knee and flying over.
Half Time: Pompey 0 Bristol Rovers 1
Mousinho made an attacking change at the break in search of a leveller, with Lowery making way for Christian Saydee.
And the Blues were soon on the attack, with a thumping Whyte effort from the edge of the box superbly saved by Cox.
The keeper did not get a firm punch on the resulting Scully corner, but Whyte’s shot was charged down.
Saydee then outmuscled a defender on the right and swung in a cross that was glanced wide by Scully.
Pompey might have fallen further behind on the hour mark, though, as Norris dived to his left to parry away a drilled Marquis strike.
Abu Kamara and Jack Sparkes were brought on as the hosts searched for a way past a Rovers side who were looking to soak up pressure.
They kept plugging away and Pack’s header was saved by Cox after the skipper had met a Sparkes delivery.
And a great chance to level came on 83 minutes, when Connor Ogilvie collected Poole’s pass and drilled a low shot against the base of the post.
Kamara could then not keep his header down from a Sparkes cross before Scully made for Yengi.
It seemed like that would be that, only for Pompey to grab an equaliser in the second minute of stoppage-time.
Kamara was clearly having his shirt tugged, but kept his compose to lay the ball off for Rafferty on the right.
The defender sent in a fine cross towards YENGI in the centre and his header hit the inside of the post before crossing the line to ensure the spoils would be shared.
Pompey (4-3-3): Norris; Rafferty, Poole, Towler (Sparkes 73), Ogilvie; Lowery (Saydee 46), Pack (c), Morrell; Whyte (Kamara 65), Bishop, Scully (Yengi 83)
Goals: Yengi 90+2
Booked: Morrell, Lowery, Towler, Poole
Subs not used: Schofield, Raggett, Devlin
Bristol Rovers (4-2-3-1): Cox; Hoole, Wilson, Connolly (Taylor 61), Gordon (Grant 65); Ward, Finley (c); Thomas (Sinclair 82), Brown, Collins (Evans 82); Marquis
Goals: Thomas 24
Booked: Finley, Wilson, Hoole
Subs not used: Belshaw, Crama, Jones
Referee: Sam Allison
Attendance: 19,165 (1,991 away fans)