Kilmarnock became only the second team to beat Hearts at Tynecastle Park in 18 months thanks to Ross Millen's winning goal.
The right-back scored after 73 minutes of his league debut to propel the Ayrshire club to within two points of the Premiership leaders.
Hearts attacked for much of the afternoon but were unable break the well-organised Kilmarnock defence. They succumbed to one of several breakaways as the game entered its closing stages.
That secured three vital points for Steve Clarke and his impressive side, who are the only Scottish side other than Celtic to win in Gorgie since May 2017. Hearts remain top of the league but Celtic sit just a point behind with two games in hand.
Having left new Scotland call-up Jamie MacDonald and the Northern Ireland internationalist Jordan Jones on the bench, there was an unusual look to Kilmarnock's team prior to kick-off. They were also without the suspended Greg Stewart, while Scotland full-back Stephen O'Donnell did not make the matchday squad.
Hearts set out to capitalise on those potentially-key omissions. They recalled striker Craig Wighton, left-back Ben Garuccio, winger Callumn Morrison and midfielder Olly Lee to their starting line-up. Within seconds of kick-off they appealed for a penalty for Mikael Ndjoli's challenge on Morrison.
However, Kilmarnock broke forward within seconds to fashion a brilliant chance. Chris Burke found himself one-on-one with the Hearts goalkeeper Zdenek Zlamal from Aaron Tshibola's through ball. The Czech spread himself to block Burke's shot with a vital save.
Another penalty claim by the hosts looked stronger when the visiting striker Kris Boyd appeared to impede Steven MacLean inside the box. Referee Don Robertson was again unmoved. Peter Haring lashed a bouncing ball wide from 25 yards to complete a frenetic opening ten minutes.
Hearts were opening Kilmarnock up with intelligent moving and passing interchanges despite Clarke organising his team in a 4-5-1 formation. Goalkeeper Daniel Bachmann, deputising for MacDonald and also making his league debut, had to deal with several cross balls into the danger zone along with his defence.
Wighton shot into the side-netting after the half-hour as Hearts' probing continued. Kilmarnock seemed content playing counter-attacking football and almost profited from that tactic on 38 minutes. Ndjoli's driving run down the left flank ended with a powerful shot which rose narrowly over the stanchion of Zlamal's post and crossbar.
Hearts' clearest scoring opportunity of the first half arrived five minutes before the interval. Arnaud Djoum won a loose ball and skipped past three opponents to slide a weighted pass left to Garuccio. The Australian's first-time effort inside the penalty box was well stopped by the outrushing Bachmann.
Boyd and Ndjoli both came close with efforts at the Hearts goal early in the second half. At the opposite end, MacLean found the net for Hearts but was flagged offside after Jimmy Dunne flicked Garuccio's cross into the striker's path.
The pattern of play didn't change much from the first period. Garuccio became more of a threat as Peter Haring supplied him with a clutch of passes from central midfield. Haring then took matters into his own hands and dispatched a 30-yarder which Bachmann needed two attempts to hold.
The breakaway was always a hazard for the home side and it was that method which earned Kilmarnock the lead on 73 minutes. Left-back Greg Taylor got forward on the left and cut the ball back to Burke, who turned and rolled into Millen's path. He drove a vicious 18-yard shot which beat Zlamal due to its power and crashed into the net.
The final stages became increasingly intense as the hosts tried to salvage something from the game. Wighton's header hit Kirk Broadfoot, prompting loud appeals for handball which were ignored by the match official. Kilmarnock's manful defending continued through to the final whistle, though, much to Hearts' disappointment.
Hearts (4-4-2): Zlamal; M Smith, Dikamona, Dunne, Garuccio (Mitchell 86); Morrison (McDonald 68), Haring, Djoum, Lee; MacLean (Clare 83), Wighton. Unused subs: Doyle, Bozanic, Hughes, Cochrane.
Kilmarnock (4-5-1): Bachmann; Millen, Broadfoot, Findlay, Taylor; Burke (McKenzie 75), Tshibola, Dicker, Power, Ndjoli (Wilson 90); K Boyd (Jones 79). Unused subs: MacDonald, Byrne, Kiltie, S Boyd.
Referee: Don Robertson.