Dundee United inflicted a major blow on Celtic’s Scottish Premiership title defence with a shock 2-0 victory at Tannadice to keep their own top six ambitions alive.
The champions now trail leaders Hearts by five points, and rivals Rangers by two, as Martin O'Neill suffered just his second domestic defeat since returning to the club.
A first goal in almost two years for Will Ferry and a first in Tangerine for January signing Emmanuel Agyei earned United a second successive home victory over Celtic for the first time since 1991.
They have closed the gap on sixth-place Falkirk to six points with two games until the split, but must also overturn a goal difference of seven if they are to secure the most unlikely of top half finishes.
Celtic huffed and puffed on the weather-beaten surface but lacked quality in the final third and were deservedly beaten by Jim Goodwin's ruthless outfit.
United went 2-0 up in the Dundee derby last week and crumbled, they did the opposite this time.
If anything, it could've been more.
The pressure was on the champions after title rivals Hearts and Rangers won their respective matches on Saturday, and it showed.
O'Neill's side were supposed to be boosted by the returns of influential captain Callum McGregor and striker Kelechi Iheanacho, but they each lasted just 56 minutes.
Despite the reintroduction of their metronome, Celtic lacked rhythm and never really looked like scoring.
Max Watters and Ferry both came close to finding an opener in the first half as the hosts looked to play on the counter with little threat coming from their passive visitors.
Kieran Tierney sliced one over and Ashley Maynard-Brewer pulled off a good save to deny Iheanacho, but that was it in terms of their first half chances.
The pitch was dry, bobbly, sandy - not exactly conducive to good football, but that does not excuse Celtic's lack of intensity.
Six minutes into the second period, Ferry punished them.
A sweeping counter resulted in the two wing-backs combining as Luca Stephenson picked out his teammate who converted at the back post.
And they didn't stop there.
Agyei added a sublime second 15 minutes later when he stepped onto a poor clearance and drilled a first-time effort into the bottom corner from the edge of the area.
Vicko Sevelj, Amar Fatah and Ferry again all endeavoured to make it three thereafter as the champions slumped to defeat.
Barring a speculative effort from Reo Hatate, which was nearly dropped into his own goal by Maynard-Brewer, and a sliding effort at a wide angle from Daizen Maeda, O'Neill's men failed to create any real opportunities.
They dominated possession, but that was the only metric in which they bettered their only opponents.
With the international break on the horizon, this sore defeat will stew in the minds of the Celtic supporters who know the title is, once again, no longer in their hands.
Seven games to go. Eighth defeat of the season.
Celtic manager Martin O'Neill on Your Site:
"We had a lot of possession in the first half, we were camped outside their penalty area but weren't able to penetrate their team and in the second half we lost our way completely. The first goal was going to be vital in the game and we probably didn't create enough and maybe we didn't adjust in the second half to conditions but that's taking nothing away from Dundee United - they deserved to win.
"The game got away from us, that's true. Goals win matches and we chased the game a little bit but, even so, we should have been able to do better. This was always going to be a difficult game for us, away here at Tannadice, I've never had an easy game here.
"It is a setback at this particular time of the season and you can't really afford to do that but we are not finished yet.
"Plenty to reflect on [over the international break]. Over the last couple of months, we have shown a really great spirit and determination to fight back in matches to go and claw late goals. The fight and determination was kind of a hallmark for us and that was missing in the second half today."
Dundee United boss Jim Goodwin on Your Site:
"It was a great result and a really good performance. I think it was no more than we deserved at the end of the game. We defended brilliantly, we spoke before the game about the disappointment of last week. We weren't resilient enough at times, but I thought today that's more like us.
"Especially in the dying moments, showing a bit of calmness, sticking together and putting the bodies on the line. My big captain Ross Graham was outstanding, alongside Iurie Iovu and Krisztian Keresztes as well. Those three, I have been critical of them, they know that, and they've accepted some of that criticism that's come their way. It's days like today that we know they're capable of, we just need to see it on a more regular basis.
"I thought we had the more dangerous attacks, more shots, more attempts on target, more corners. When you look at the whole picture, I think we deserved the three points today.
"Hopefully we can win the next two and Falkirk don't pick up any points and we can force our way into that top six but it's probably going to be one of those seasons we look back on with regret because we've thrown away so many points from winning positions that would put us into that top half.
"Now is not the time for being negative, credit to the players, along with myself, we've had a lot of criticism and deservedly so. We're sitting seventh, we're not down near the bottom of the table fighting a relegation battle. People can see what we're trying to do here, we just have to build on it."
