Pompey conceded a late goal, as their unbeaten opening to the 2020/21 campaign was ended at Chichester & Selsey.
They recovered from falling behind to level through a fine Becki Bath free-kick and dominated much of the contest.
The Blues had chances to score a winner, but Amber Howden’s effort ensured they departed with nothing.
Jay Sadler made two changes from the side that beat Keynsham at the weekend, with Eilidh Currie and Tamsin de Bunsen replacing Jazmin Younger and Jade Widdows.
The visitors had an early opportunity to break the deadlock, with Issy Foster called into action to turn behind Katie James’ effort.
They then had strong appeals for a penalty turned down following a challenge inside the area on de Bunsen.
And when Rosie McDonnell sent a teasing delivery into the centre, it was just out of reach for the onrushing James to meet.
Instead it was Chichester who broke the deadlock against the run of play on 33 minutes, as they stole possession, allowing Megan Fox to charge forward and slot past Hannah Haughton.
Half Time: Chichester & Selsey 1 Pompey 0
Pompey were back on the attack when the action restarted and Danielle Rowe forced a save from Foster.
Ava Rowbotham then saw a volleyed attempt blocked, as the Blues continued to look for a breakthrough.
And they did have a leveller just before the hour mark, courtesy of a fine free-kick from BATH that flew into the net.
The visitors almost went in front moments later, but this time Bath’s effort was tipped over the bar by Foster.
Chichester then had to make do without one of their players for 10 minutes, with the referee making use of the ‘sin bin’ ruling to penalise Jane Yeates for dissent.
Kat Cunningham came on for the closing stages and almost snatched a late winner, dragging a shot wide after being teed-up by McDonnell.
But instead it was their opponents who stole all three points at the death, when Howden reacted quickest following a scramble in the box to smash the ball home.
There was still time for the Blues to try to force an equaliser, but Rowbotham and McDonnell both fired narrowly off target to ensure a first defeat of the season.
Pompey: Haughton; Tonks, Currie, Rowe, McDonnell; Bradley (c), Rowbotham, James; Bridge (Widdows 71), de Bunsen (Cunningham 83), Bath
Subs not used: Ingram, Mason, Adaway