Sheffield United moved into the automatic promotion places as Chris Basham's strike was enough to secure a vital 1-0 win and leapfrog Leeds United at Elland Road.
The Blades withstood heavy pressure in the first half as Tyler Roberts and Patrick Bamford both went close, but a seventh clean sheet in a row laid the foundations to a third straight victory for the in-form Blades.
None have felt as significant as this in the race for promotion, with Basham's strike 19 minutes from time meaning Sheff Utd have moved one point clear of Leeds, who ended the game with 10 men after Kiko Casilla was shown a red card in stoppage time for a professional foul on Billy Sharp.
The result means Marcelo Bielsa's side missed the chance to go five points clear of their Yorkshire rivals with eight league games remaining in the Sky Bet Championship.
Leeds: Casilla (5), Ayling (6), Alioski (6), Cooper (6), Jansson (7), Phillips (7), Roberts (6), Klich (6), Harrison (6), Hernandez (7), Bamford (6).
Subs: Douglas (6), Dallas (6), Clarke (7)
Sheffield United: Henderson (7), Baldock (7), Stevens (7), Cranie (7),O'Connell (7), Egan (7), Basham (8), Fleck (7), Norwood (7), McGoldrick (7), Sharp (7).
Subs: Lundstram (n/a), Dowell (n/a)
Man of the match: Chris Basham.
Leeds dominated much of the first half and had the better chances on a saturated pitch.
Bamford looked set to score from a Pablo Hernandez pull back inside eight minutes had it not been for a block, with Jack Harrison left holding his head in his hands soon after having turned over.
Leeds were probing and Bamford tried his luck from an acute angle, before Roberts headed just wide.
Each missed chance led Bielsa to become increasingly agitated and offered hope to the Blades, who had struggled to get a foothold on proceedings before ending the half on top.
Leeds, who had won their last three, made no changes to their sides as expected. It meant Patrick Bamford continued to lead the line in the absence of Kemar Roofe.
Chris Wilder made three changes and they were all at the front as out went Kieran Dowell, Scott Hogan and Gary Madine and in came David McGoldrick, Martin Cranie and Billy Sharp.
While Leeds survived a penalty-box melee, David McGoldrick and Sharp had chances before Basham lost his composure after rounding Leeds goalkeeper Casilla.
The second half was a far edgier affair, with Roberts eventually sparking things into life by rattling the outside of the post in the 59th minute. But just as the Blades looked to be creaking, they landed a hammer blow at the other end.
Ponderous defending by Liam Cooper allowed Sharp to collect the ball and play in Basham, whose strike from the edge of the box found the bottom left-hand corner.
Leeds' response to the 71st-minute setback was predictably gung-ho and the best opportunity fell to Pontus Jansson five minutes from time.
Struggling to move due to injury, the defender was deployed up front - with all three Leeds substitutions made - and turned agonisingly wide.
There was more drama to come as Sharp burst through in stoppage time and was taken to ground by Casilla, who was sent off for the challenge. Fan favourite Jansson donned the gloves but there was to be no more drama.
Marcelo Bielsa said: "It was a really important game and I don't want to underestimate what the opponent got. If we analyse the result, it is what it is and if we analyse who deserves what, we believe we deserved something.
"I won't make an opinion on the sending off. I'll keep my thoughts to myself."
Chris Wilder said: "We've done to Leeds what they've done to us on more than one occasion since our introduction back into the Championship. We stuck in there and have a tremendous attitude to keep us in the game to perhaps win it. Fortunately, we did that.
"Bash scored and I said to him he was lucky to still be on as it was nearly a Bielsa-style substitution after 20 minutes! That's the character of the boy as well as the group. He dug himself out of a hole and got the winner for us."
Man of the match - Chris Basham
Chris Basham scored the all-important goal, but he made a vital interception in the first half to deny Patrick Bamford, before his energy led to him offering support to Billy Sharp for his strike.
He told Sky Sports: "The gaffer told me I ought to have shot in the first half, so I thought to myself, I never get in those positions. It was a 'fish-out-of-water' moment, but in the second half I just slid and shot. It's a massive win for us and three more points to where we want to be.
"The first 20 minutes I wasn't my normal self but these things happen when you go to Elland Road. We've kept seven clean sheets now and everyone is buzzing."