Nottingham Forest moved to within one point of Leeds after sealing a 2-0 victory over Marcelo Bielsa's side at the City Ground.
Leeds knew victory at Forest would see them move to the top of the Championship - at least until Sunday - but Sammy Ameobi's 31st-minute strike swerved past Kiko Casilla at his near post to give the hosts the lead.
Lewis Grabban then wasted a glorious opportunity to extend Forest's advantage early in the second half, firing a shot straight at Casilla with plenty of the goal to aim at, while Brice Samba clawed the ball off his own goal line to keep out Liam Cooper's header late in the game as Sabri Lamouchi's held onto their advantage.
They finally made the game safe deep into stoppage time through Tyler Walker (90+4), securing what could prove to be a crucial three points in the race for promotion to the Premier League.
Victory sees Forest move to within two points of league leaders West Brom, who have the chance to extend their lead at the top when they travel to Millwall, live on Sky Sports on Sunday.
However, it's still very congested at the top following wins for Forest, Fulham and Brentford on Saturday, with the top seven teams separated by six points as this enthralling race for automatic promotion continues.
How Forest stunned Leeds...
Forest enjoyed a fast start with Grabban scuffing an effort wide in the seventh minute, before Tiago Silva had a long-range effort saved by Casilla three minutes later.
Referee Oliver Langford waved away strong shouts for a home penalty after Cooper appeared to tug back Joe Worrall following a corner.
Silva was again thwarted by the Leeds shot-stopper with another long-range effort in the 13th minute as Forest stepped up the pressure.
Patrick Bamford made a mess of a header from a Pablo Hernandez cross, before Ezgjan Alioski had a shot deflected out for a corner.
Ameobi fired Forest when he beat Casilla at his near post, extending Leeds' unwanted record of having gone behind in every game since December 14.
Hernandez had a shot easily blocked on the stroke of half-time as the visitors continued to look flat up front, before Grabban missed his sitter - shooting straight at a prone Casilla after Forest had caught Leeds on the counter-attack.
Silva had a shot well-blocked by Ben White as he looked to guide the ball into the bottom corner in the 64th minute.
An unmarked Copper directed a powerful header straight at the keeper from a 75th minute corner, before the Forest keeper palmed the loose ball away on the goal-line.
Walker wrapped up the win in the fourth minute of stoppage time, tapping in after Joe Lolley had set him up after robbing Harrison of the ball.
Faltering Leeds, who were 11 points clear of third place earlier in the season, have won just one of their last nine Championship games and are now level on points with Fulham.
Man of the match- Joe Lolley
What the managers said…
Nottingham Forest boss Sabri Lamouchi: "It was a massive result and a massive performance against a fantastic team. Leeds made more mistakes than usual, but we disturbed them without the ball. My players played with fight and determination and they played so well today.
"The players made me so proud and happy. We have 15 more games and we play again in three days. This league is crazy, and you must be ready each game. I have a fantastic group of players, it is very easy to work with this club and these players.
"My goal was to finish the season, the last time a manager did that was 11 or 12 seasons ago. We want to get to the end of the season and see where we are. I am happy for the players. When you play with this focus, attention and temperament, you can believe in the quality you have."
Leeds boss Marcelo Bielsa: "The responsibility for defeat is on me and not the players. The players gave me everything. The team gave a big effort, but it was not enough. It is not necessary to offer a justification at this stage for the defeat. The players gave maximum effort.
"If a manager has human resources and they give him everything, then the responsibility is on the manager. It is down to me to do our best and change this tendency from the next match. It is not a matter of blaming individual players. It is not about an individual's effort that we have seen today. Maybe there are some specific details that we can correct. If the manager has a very good player that gives his all for the team, he has to take the responsibility and look for new solutions in the next match. I have to take some tough decisions."
Leeds face another tricky looking trip to a promotion rival on Tuesday when they travel to Griffin Park to face Brentford (kick-off 7.45pm), live on Sky Sports Football Red Button from 7.40pm.
Meanwhile, Nottingham Forest can push their promotion claims further when they face Charlton at the City Ground (kick-off 7.45pm), also on Tuesday on the Sky Sports Football Red Button from 7.40pm.