Oscar Schwartau came off the bench to fire Norwich to a 2-1 win over Millwall - and do their fierce rivals Ipswich a big favour.
The Lions, who started Easter Monday in second, had taken the lead through substitute Mihailo Ivanovic shortly after half-time.
But a long-range rocket from Pelle Mattsson and Schwartau's winner condemned them to defeat to give the chasing pack - including Norwich's East Anglian foes Ipswich - a major boost in the race for automatic promotion.
Norwich, who now have seven wins from their last nine away league matches to keep their slim play-off hopes alive, started quickly and Anis Ben Slimane saw a shot deflected wide with just 60 seconds on the clock.
Mathias Kvistgaarden and Ali Ahmed also had shots blocked with Millwall's defenders throwing themselves in front of the ball, just as they had in the huge Good Friday win at Middlesbrough.
When Slimane did manage to break through the shackles his shot was easily dealt with by goalkeeper Anthony Patterson.
Norwich had two golden chances to open the scoring, first when Jack Stacey raced clear down the right only to drag his shot across goal and just the wrong side of the far post.
Then Kenny McLean met Ahmed's cross but could not divert the ball into the net.
It was a tame first half from the Lions, who only threatened when Femi Azeez wriggled past a couple of challenges only to fire off target and Tristan Crama flashed a late header wide.
McLean had a heated discussion with Millwall boss Alex Neil, who used to manage Norwich, just before half-time to at least give the home crowd something to get animated about.
Neil sent on Ivanovic for the ineffective Luke Cundle and the move paid dividends 11 minutes into the second half.
Crama played a delightful ball out to Casper de Norre, whose cross found the head of Ivanovic to guide home from four yards.
But eight minutes later Norwich hauled themselves level when the ball was worked to Mattsson 20 yards out and the Dane crashed his first-time shot into the top corner.
Then, in the 76th minute, Norwich broke forward three-on-two and Mohamed Toure kept his nerve to play in fellow sub Schwartau for a simple finish past the exposed Patterson to win it for City.
Millwall's Alex Neil:
"The problem we had was energy in the middle of the pitch. We looked a bit jaded, we didn't look our normal selves.
"We didn't move the ball well enough. I don't know whether it's nerves or a lack of quality, but I thought Norwich were better.
"We managed to get the first goal and I think my biggest disappointment is we are usually good from that position.
"It's a disappointing day. I don't think over the piece we did enough to win the game and we didn't defend two moments well enough."
Norwich's Philippe Clement:
"I just said to the players in the dressing room that for me it's been our best performance.
"I have huge respect for what Millwall and what the manager here are doing this season and what a tough place this is to come.
"If you don't have enough movement against this team you get smacked.
"To be that dominant and to be the better team shows good signs. I challenged them again this morning and they responded."
