Andre Brooks scored a last-minute winner for Sheffield United who denied Portsmouth a valuable point in their fight for Championship survival.
Pompey created numerous chances in the game at Fratton Park but were left with nothing to show for their performance, after Brooks' close-range finish.
Both sides threatened to break the deadlock in an entertaining first half, with Gustavo Hamer hitting the woodwork for the Blades before Pompey's Colby Bishop had a goal ruled out.
Numerous opportunities presented themselves at both ends after the interval, with Brooks eventually coming up with the 90th-minute winner for Chris Wilder's side.
Still, it took a double save from Blades' goalkeeper Michael Cooper in the sixth minute of stoppage time to deny Millenic Alli a late leveller.
Defeats for both sides in their previous games prompted changes for the tie at Fratton Park, with Kalvin Phillips given his first start for the Blades after his loan arrival from Manchester City.
The closest his side went to breaking the deadlock in the first half was Hamer's 15th-minute shot from a tight angle that hit the foot of the post.
And while Pompey's Bishop managed to find the back of the net from close range soon after, home fans were left incensed when their celebrations were cut short after the finish was ruled out because of a foul in the build-up.
The decision only added to Pompey's frustrations in a first half in which Adrian Segecic and Zak Swanson tested Blades' keeper Cooper.
John Swift and Alli threatened for the hosts in the second half, while Patrick Bamford called Nicolas Schmid into action and Brooks sent an effort just wide at the other end.
As the game was seemingly heading towards a stalemate, Tyrese Campbell did well to squeeze a cross to the back post for Brooks to score his fourth goal in five games to win it for the visitors.
Portsmouth manager John Mousinho told to BBC Radio Solent:
"Any loss is difficult to take, but the nature of the goal conceded and the way the game panned out makes it really difficult.
"The games where you come away and know you are not the better side, there are concerns when that happens and you can think 'we weren't at it today or not good enough'. But I thought we were the better side today and should have won the game.
"The opportunities that were created and some of chances we carved out were excellent, but we didn't show enough composure and ruthlessness in those moments."
"For a very dominant first-half display, we didn't come in with a goal. That's fine it happens, but it was about building on that in the second half and we probably didn't do that until the last 25 minutes when we settled into the game again and had a few opportunities.
"But ultimately we got caught in the end."
Sheffield United boss Chris Wilder told BBC Radio Sheffield:
"It was mentality maybe over control and us imposing ourselves. They were better at what they did in the first half than what we did, but they never put us to bed.
"We grew into the game in the second half. At half-time we talked about it, that we could be a little bit better and I'm not so sure they could do any better than that.
"We knew the game would settle down a little bit and it did, and we started to impose ourselves.
"I thought Kalvin [Phillips] in the middle of the park, it was a really tough one to get chucked into because of the pace and the nature of the game and the intensity of it. They had two midfielder players who were all over our midfield players, and they won that battle in the first half. But Kalvin gave us an opportunity to start finding passes and getting out, while always knowing the opposition are dangerous.
"Maybe I would have taken a point with 10 minutes to go, and that not being negative because this is a difficult place to come. So to get a winner is brilliant for everybody."