Sheffield United's leading scorer Gustavo Hamer put them ahead with his seventh Championship goal of the season
Sheffield United edged past strugglers Portsmouth despite an unconvincing display to move back into the Championship automatic promotion places.
The Blades took a 24th-minute lead when Ben Brereton Diaz released Gustavo Hamer, who drove into the box and curled a superb low finish inside the right-hand post.
But Pompey deservedly equalised just three minutes later when Josh Murphy sent in a dangerous low cross which full-back Connor Ogilvie glanced in at the near post.
The visitors continued to threaten after the break, but were hit by a sucker-punch when Rhian Brewster threaded a low cross into the six-yard box for fellow substitute Jesurun Rak-Sakyi to slide home the winner.
With both of the top two playing in the FA Cup, Chris Wilder's side took advantage as a fifth win in six league outings moved them above Burnley into second place, and within two points of leaders Leeds.
Portsmouth could easily have been in front themselves before the 73rd-minute winner after spurning a string of good chances, but a seventh straight away league defeat leaves them two points above the bottom three, in 20th.
Colby Bishop was denied by United goalkeeper Michael Cooper when clean through, and Callum Lang guided Murphy's left-wing cross over the bar from point-blank range before Hamer put the hosts ahead with his seventh goal of the season.
The Blades struggled to handle Murphy, who teed up Pompey's swift reply and then curled a shot against the crossbar.
Ogilvie missed a terrific chance to put the visitors in front when he diverted another dangerous Murphy cross over the bar from close range, and moments later Portsmouth keeper Nicolas Schmid had to turn Harry Clarke's header around the post.
Portsmouth continued to threaten a first win at Bramall Lane since 1955 after the break, with an offside flag denying Bishop a goal when he flicked in Murphy's cross.
Cooper did well to parry away Murphy's low shot and then tip over another effort for the livewire midfielder, but the Blades' strength off the bench would prove the difference.
Just two minutes after coming on, Brewster brilliantly teed up the matchwinner for Rak-Sakyi, and then almost made it three in stoppage time when through one-on-one, but smacked a shot against the bar.
Sheffield United boss Chris Wilder told BBC Radio Sheffield:
"We weren't very good today and it's as simple as that. It was one we escaped with because we have the ability to win a game of football when we don't play very well.
"They were better than us in all departments, first and second balls. It wasn't anything about tactics today, it was about an attitude towards the game which I told them at half-time.
"We looked to short-cut the game, which is really surprising, and we won't do that again.
"The reaction in the second half was better, even though it wasn't to my liking, but there's a little bit of arrogance about us which I talked about at half-time and we can't afford to let that happen."
Portsmouth manager John Mousinho told BBC Radio Solent:
"It's really difficult to take considering the performance and how well the boys played, how many chances we created, and how frustrated we made Sheffield United throughout the 90 minutes.
"The only criticism I have of the boys is that we didn't put the ball in the back of the net more, that we weren't clinical enough in some of those moments.
"I feel very positive about the performance now. We've been standing here after plenty of away games where the performance hasn't been good enough, the result's been dreadful and we've got what we deserved. Today we didn't get what we deserved."
