Jacques Maghoma's winner was his fifth goal of the season for Birmingham
Birmingham City moved within sight of Championship survival as victory over Sheffield United lifted them five points clear of the relegation zone.
Mark Duffy had fired the Blades ahead, capping a break by beating home keeper David Stockdale at his near post.
Marc Roberts levelled for Blues before the interval, hammering in Jacques Maghoma's corner from close range.
And Maghoma slid home the winner from 15 yards to leave Sheffield United three points off a play-off place.
However, the Blades' biggest issue between them and overhauling sixth-placed Millwall after losing at St Andrew's is the fact that there are now four other teams between them in 11th and the Lions.
But for Blues, this victory leaves them with their fate in their own hands after relegation rivals Bolton and Barnsley were beaten by champions Wolves and Leeds respectively.
Victory for the Tykes in their game in hand at Nottingham Forest on Tuesday would cut Birmingham's buffer to three points and send Bolton, who host second-from-bottom Burton next Saturday, into the bottom three.
However, anything less would mean that Garry Monk's side would be safe if they pick up another win at QPR next weekend.
The mood at St Andrew's was certainly positive as Duffy's early strike was answered by the home support with a defiant rendition of Blues' anthem Keep Right On.
Their side responded when Roberts was perfectly placed to net when Lukas Jutkiewicz flicked on Maghoma's corner following a spell of home pressure.
And although the Blades poured forward after Maghoma had slotted home from Che Adams' pass, Birmingham held on with relative ease.
Birmingham City boss Garry Monk:
"It was a huge step forward in our situation but it is definitely not job done although the other results went our way.
"To go a goal behind in the Championship is hard enough. But it was great to see the mentality and belief of the players.
"I was not happy conceding the early goal but the response is exactly what I've been seeking.
"We are definitely a step closer to safety but the only people who are going to get us out of this situation is ourselves."
Sheffield United's boss Chris Wilder:
"The only bright part of the game for me was the start.
"We just didn't play good enough individually all over the pitch. We lost too many balls and made some really poor decisions.
"We didn't deserve to win as Birmingham did enough to take the points and that is all that matters.
"We had a great opportunity to achieve something this season. But we have not come up with big enough moments when we were doing well. That is the difference between us the teams who are in the play-offs."
