Manchester United suffered a shock 2-1 home defeat by the Premier League's bottom club Sheffield United to leave Manchester City alone at the top of the table.
Substitute Oliver Burke grabbed the 74th-minute winner after Harry Maguire had cancelled out Kean Bryan's first-half opener for the Yorkshire side who had won just one league game all season before their trip to Old Trafford.
City are top on 41 points, a point ahead of United who have played one game more.
Chris Wilder's side went into the game with the worst record, after 19 matches, in Premier League history and without a league win at Old Trafford in 48 years while United were unbeaten in 13 league matches.
But with 38-year-old former Everton defender Phil Jagielka marshalling their defence, Sheffield United showed exactly the kind of determination and fight they will need if they are to somehow get out of trouble.
Ole Gunnar Solskjaer opted to rotate his team, leaving out Fred and Scott McTominay from central midfield and that had a big impact on the game with United lacking the intensity that pair have been providing in the middle of the pitch.
Sheffield United, who now have eight points, went ahead in the 23rd minute when Bryan headed in John Fleck's corner with United keeper David De Gea flapping at the cross, although replays suggested he may have been pushed by Sheffield striker Billy Sharp.
United's frustration over that decision was amplified when Anthony Martial had the ball in the net but the effort was ruled out for a foul by Harry Maguire on keeper Aaron Ramsdale.
NO ZIP
Solskjaer's side were lacking any of the zip and invention of their recent good run of form but managed to get on level terms when their Sheffield born captain Harry Maguire met an Alex Telles corner with an unstoppable header.
United paid the price from hesitant defending, however, when Burke's low drive deflected off Axel Tuanzebe and crashed in off the underside of the crossbar.
Solskjaer demanded his team pick themselves up for their trip to Arsenal on Saturday after suffering their first league defeat since early November.
"Disappointed, we couldn't get to the heights of the last few months' football we played. Very disappointing, we couldn't find a solution to their very compact block and good defence.
"That little spark, that edge, we didn't have that extra creativity, imagination, the X factor we've had - it's happened, we look at it, agree it is not good enough and go again," he said.
"There is no time to feel sorry for yourself because Saturday you go again, we have Arsenal, we have to forget this, learn from it and move on."
Burke was ecstatic at his club's first win at Old Trafford since 1973.
"Amazing result, really was a must-win for us and I'm chuffed for the boys, they put in a great shift. Amazing feeling and amazing for me to get the goal, I am buzzing for the boys and hopefully we can take that into our next games," he said.
"We showed great courage and great determination. We show great character and never stop, every game we have showed great football but just not taken three points."