Leeds were knocked off the top of the Championship table following a 2-0 defeat at home to Sheffield Wednesday.
Former United boss Garry Monk returned to thwart his ex-employees as Jacob Murphy and Atdhe Nuhiu hit late goals.
Jack Harrison missed two good chances and Patrick Bamford had a goal ruled out for offside as Leeds enjoyed the better of the first half. But they found the second half tougher going as Wednesday successfully stifled their creativity.
The South Yorkshire side were given their rewards when Murphy beat Kiko Casilla from a narrow angle. Nuhiu wrapped the points up four minutes into stoppage time after Adam Reach teed him up. It left Leeds six points ahead of third-placed Brentford in the race for automatic promotion.
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Luke Ayling and Helder Costa both had chances for the hosts inside the first 10 minutes. Massimo Luongo headed well wide from a Barry Bannan corner after 10 minutes with a free header.
Bamford failed to get hold of his shot after he had won the ball back, while Murphy fired a first-time shot wide after Liam Cooper did enough to put him off. Harrison volleyed wide from close range when Kalvin Phillips picked him out.
Bamford had a header chalked off five minutes later after Mateusz Klich was ruled offside in the build-up. Cameron Dawson brilliantly tipped a Harrison shot around the post in the 27th minute, before Julian Borner failed to keep a 20-yard half-volley down.
Dawson saved a Bamford shot at his near post and Costa smashed over in stoppage time. Bamford shot over three minutes after the restart after Costa got to the byline and picked him out.
Sam Winnall just failed to get on the end of Murphy's ball across goal a minute later and then fired a brilliant chance over after a Morgan Fox header fell kindly. Costa failed to find the target as Leeds were once again restricted to shots from long range.
Dawson saved a curling effort from Klich four minutes later, before Casilla had to turn away Murphy's goalbound inswinging corner. Reach fired over with a cross-shot in the 74th minute, before Harrison missed the target with a header with 10 minutes to go.
Murphy's low shot squirmed past Casilla in the 87th minute to give Wednesday the lead, and it was game over when Reach got in behind the defence and set up Nuhiu deep into stoppage time.
What the managers said...
Leeds' Marcelo Bielsa: "Clearly this was a match which was very important to make a difference. It is a defeat, but it is not a definitive match. It's a moment which was very important to get a good result. We controlled the game and missed a lot of chances but there are still loads of games to go.
"We need to look at the collective role that the team played in conceding possession when they scored their goals rather than blame any individuals. I can understand the anxiety amongst the crowd when key moments to win the game do not go as we want. There is a sense of doubt around the team when things are not going to plan, and I do not mean the players or fans."
Sheffield Wednesday's Garry Monk: "It was very difficult for us facing the league leaders away from home. Our game plan was good. We had to deal with the pressure and to take our chances when they came, we did that really well.
"You want to see your players play with heart after the week we had. When you play with real heart, you have the courage on the pitch. You feel it and that's exactly what we needed. We matched them and we took our chances. We have played with heart and courage in our last two games. When you play like that you will get your rewards, we were good value for the win."