Leaders Hearts dropped three valuable points in the Scottish Premiership title race as they lost 1-0 to an inspired Kilmarnock at Rugby Park.
Michael Schjønning-Larsen scored the only goal of the game as Kilmarnock eased their own relegation fears while blowing the title race wide open.
Hearts had watched to move to within two points of the league leaders and knew they had to take maximum points in Ayrshire to wrestle the momentum back, with third-placed Rangers travelling to Paisley on Sunday to face St Mirren.
Instead, it was Kilmarnock who were good for the three points and gave their own survival hopes a serious shot in the arm, with this win moving them level on points with St Mirren in 10th place and nine clear of basement side Livingston, who they play next week.
The away side started brightly and got their first sight of goal just a minute in when Claudio Braga got on the end of a Blair Spittal cross, but his header was gathered by Kelle Roos.
Kilmarnock were undeterred, however, and applied some early pressure of their own. Findlay Curtis went close when his deflected effort from the edge of the box crashed off the post.
Braga looked as though he was in the mood, seeing a volley deflected out for a corner before trying a neat flick from that delivery that just went wide of the post.
At the other end, Curtis was a menace to the Hearts defence, getting away from Michael Steinwender and delivering a cross for David Watson, but his blocked shot was weak.
Kilmarnock took the lead after 17 minutes when Greg Kiltie's outstanding cross was met by Schjønning-Larsen, who ghosted in at the back post to tap the ball past Alexander Schwolow.
Hearts looked to hit back quickly but Roos was on hand to deny Braga's powerful left-footed effort before Tomas Magnusson fired an effort wide from the edge of the box.
It was Kilmarnock's turn to threaten next with Schwolow called into action twice in quick succession to deny Joe Hugill and then Curtis from range.
Kilmarnock started the second half on the front foot, with Curtis and Kiltie having decent opportunities to double their lead.
Hearts did gain a foothold in the game as the half wore on but they were creating little with their possession. Spittal called Roos into action with a shot from range, but forward incursions invariably broke down around the Kilmarnock box.
Harry Milne did manage to burst into the area but chose to fire the ball at Roos instead of cutting it back for Braga and the hosts were able to clear the danger.
Curtis again threatened Schwolow after a mazy run, but he failed to hit the target as Hearts huffed and puffed without ever seriously threatening the Kilmarnock goal, meekly falling to three defeats in a row on the road.
Kilmarnock boss Neil McCann on Your Site:
"I wouldn't say it was a complete performance; I think it was a brilliant performance. We had to come up with a plan that was respectful to the league leaders because Hearts have been superb. All in all, the biggest thing we needed to win this game tonight was matching Hearts' work rate and I thought tonight we were every bit as good as them.
"I feel at Rugby Park we are encouraging the players to be brave, tonight I am so pleased we have got a clean sheet against a free scoring Hearts side. Tonight, we just enjoy the victory, we have done nothing yet but secure a brilliant three points at home. I'm so proud of the boys tonight because it is such a difficult game."
On preparing for Livingston clash next weekend: "I'm not going to look at the Livingston match tonight and think about how the game is going to go next week. What I've said to the boys in there is get out and enjoy your victory tonight.
"You have to enjoy your wins. We have stewed on a really bad defeat to Falkirk for two weeks and it has been sore and they have reacted and bounced back brilliantly tonight.
"I'm going to let them just enjoy it and let that win sink in over the weekend and we'll get them in on Monday and prepare for Livingston next week."
Hearts head coach Derek McInnes on Your Site:
"We didn't do enough; I thought our start was poor in the game. Credit to Kilmarnock, well done to them, I thought they fought and scrapped for their win.
"We had way more possession, more shots, more territory, but we didn't really look as convincing with our work tonight and we've got to be honest with that.
"I did say there would be twists and turns but we've got to look after our own performance. We just looked a wee bit unconvincing for whatever reason, the pitch here can be difficult, it can make you a bit uneasy.
"We need to be a wee bit more aggressive, far more aggressive, I don't think we were aggressive enough in the first half.
"We'll wake up tomorrow morning, we're still top of the league, we've still got so much to play for. I don't expect this to be a pattern, I still expect us to be in the fight and we'll go again."
