Second-top Hearts slipped up as goals from Danny Invincibile and Gary Wales gave Kilmarnock a surprise victory.
Danny Invincibile opened the scoring
Steven Pressley stumbled as Steven Naismith set up Invincibile and the Australian fired in the opener.
Mirsad Beslija was next to lose his footing in the lead up to the second as Wales' effort was deflected home.
Naismith struck the crossbar before Hearts pressed forward after the break but could not avoid a defeat that leaves them eight points behind Celtic.
Hearts head coach Valdas Ivanauskas had made four changes as he continued his squad rotation policy.
Robbie Neilson returned at right-back, while there were also recalls for Beslija, Roman Bednar and fit-again Edgaras Jankauskas.
Andrius Velicka, Jamie Mole and Marius Zaliukas dropped to the bench, while Saul Mikoliunas was suspended.
Fit-again Gordon Greer replaced Simon Ford in a Kilmarnock defence that was quickly under pressure.
Deividas Cesnauskis' clever flick set Jankauskas clear and only a deflection off a Kilmarnock defender stopped the Lithuanian opening the scoring, while Beslija also had an effort deflected wide.
Gary Wales added the second goal
Killie rode out the initial storm and James Fowler shot over the Hearts bar from the edge of the box when he should have done better.
And the visitors took the lead when Invincibile finished beautifully after being fed by Naismith, who had burst forward from midfield.
His path made easier by a slip on the greasy surface from Hearts skipper Pressley and a slip by Beslija led to Kilmarnock's second goal.
Former Hearts striker Wales' effort was deflected off Christophe Berra past a flat-footed Craig Gordon.
Paul Hartley tried to haul the home side back into the game, but goalkeeper Graeme Smith beat out his free-kick and dived on the loose ball with several attackers waiting to pounce.
Killie almost made it 3-0 when Wales nodded Garry Hay's free-kick back across goal and Naismith headed against the crossbar.
Bednar spun intelligently in the box and fired an effort just wide of Smith's left-hand post as Hearts threatened for the first time in the second half.
Peter Leven should have made it three when he collected David Fernandez's flick six yards out but could only poke the ball weakly at Gordon.
Hearts winger Cesnauskis produced a powerful drive, but Smith was equal to it in the Killie goal as the points were secured by the Ayrshire side.
Hearts: Gordon, Neilson, Pressley, Berra, Wallace (Aguiar 66), Beslija (McCann 62), Hartley, Tall, Cesnauskis, Bednar, Jankauskas (Velicka 56).
Subs Not Used: Banks, Fyssas, Zaliukas, Mole.
Booked: Jankauskas, Neilson, Bednar, Berra.
Kilmarnock: Smith, Fowler, Greer, Grant Murray, Hay, Naismith, Johnston, Frazer Wright, Fernandez (Di Giacomo 90), Wales (Leven 59), Invincibile.
Subs Not Used: Combe, Dodds, Stephen Murray, O'Leary, Adams.
Booked: Frazer Wright, Greer, Leven.
Goals: Invincibile 28, Wales 35.
Ref: K Clark