Paddy Reading struck a stunning late winner as Ayr United gained a storming Ayrshire derby win to drive a hole in the title hopes of their bitter rivals.
The full-back crashed home a low half-volley following a period of constant pressure as United gained a deserved three points for a brilliant second-half show.
And it piles more pressure on Kilmarnock, now five points behind leaders Arbroath, as their stuttering bid to return to the Premiership at the first attempt took another hit.
Kilmarnock boss Derek McInnes had rung the changes from Friday night's damaging defeat at Arbroath with four fresh faces in for derby night.
Oli Shaw came in to partner Kyle Lafferty up top while veteran Chris Burke – who tormented Ayr in the first game between the sides at Rugby Park – also returned.
United head coach Lee Bullen stuck with the side that was held by Dunfermline on Saturday, with three of his January signings handed their first taste of the derby atmosphere.
And it was the home side who got the perfect start in just seven minutes as the Ayr defence was posted missing when Burke broke down the right, and his centre was slammed home by Fraser Murray.
But the lead didn't last long as Ayr levelled from a corner turned goalwards by Sean McGinty and James Maxwell got the finishing touch past a static Zach Hemming.
Killie tried to get going again and Rory McKenzie fired over from distance as they looked to get back on top.
Ayr then relied on keeper Aidan McAdams to pull off a wonder save from McKenzie before following up with a great diving stop to deny Lafferty seconds later.
And the Northern Irishman almost produced the spectacular with an overhead kick which flew just wide as Killie cranked it up.
At the other end, Hemming did well to get down and save low from Kerr McInroy as Ayr also threatened.
And it was the Honest Men who should have hit the front early in the second half when Maxwell played in Mark McKenzie but he sent his effort against the sidenetting with just Hemmings to beat.
Ayr were almost punished at the other end when Lafferty got up to send a header off the post as the game swung in both directions.
Killie survived loud calls for a penalty when the ball looked to strike Brandon Haunstrup's arm in the box but ref Bobby Madden remained unmoved.
At the other end, McAdams produced another good stop to get down low and deny Burke as the contest fizzed back and forth.
United sensed the game was there for them and sent on top scorer Tomi Adeloye from the bench – and he almost put them in front with a lob which drifted just over Hemming's bar.
The visitors then enjoyed their best spell of the game as they poured forward with Adeloye proving a real focal point.
He was inches away with a flying header from Jordan Houston's cross before Muirhead nodded just wide from a corner.
But Ayr finally got the goal their pressure deserved when the ball dropped to Reading just inside the box and he struck a sweet half volley beyond Hemmings to send the Ayr support wild.
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