There was a growing sense of frustration as Pompey failed to break down their stubborn opponents from Burton.
But John Mousinho believed that they could claim all three points – and was rewarded for his faith deep into stoppage-time.
The cross from Connor Ogilvie was nodded down by Joe Pigott for Dane Scarlett to fire home the winner.
And Mousinho was delighted that Pompey’s patience paid off to ensure most inside Fratton Park could go home happy.
The Blues boss said: “I’m absolutely thrilled with the three points and also the way the lads performed, in the second half especially.
“It was really tough – Burton had a game-plan and they executed it well. They came here on the back of three successive wins and you could see why because they’re so organised.
“Josh made one save shortly after the break, but we were largely dominant, without quite having enough in the final third. That’s something we need to work on.
“I thought we’d have one more chance and that was the whole point of getting Joe on. We wanted to keep getting crosses into the box.
"I’m absolutely thrilled with the three points and also the way the lads performed, in the second half especially."
“There had been lot of them and we didn’t have a lot of joy from it, but he did a great job of knocking the ball down for Dane and it was a brilliant finish.”
Paddy Lane and Reeco Hackett were among four changes to the side, while the injured Marlon Pack and Di’Shon Bernard both dropped out.
Mousinho said: “Reeco has really made an impact the last two times he’s come on and he got his goal at the weekend.
“He’s got his head down, hasn’t sulked and has been working hard in training. We saw what a threat he was when he found pockets of space in the first half.
“We played Paddy just off the centre-forward and thought he’d get a lot of joy there. It was nice to give him a first start.
“Marlon and Di’Shon both failed late fitness tests and so the team we thought would play on Monday had changed.
“It’s nothing serious and we’ll see how they are for the weekend, along with Matt Macey, whose illness has improved.
“We told all the boys they had to be ready – whether starting or coming off the bench – and that’s what the squad is for.”