Matty Kennedy scored a late winner as Kilmarnock inflicted a second defeat on Celtic this season.
The Hoops started in dominant fashion in Ayrshire as they looked to avenge their Viaplay Cup loss and Matt O’Riley gave them a 33rd-minute lead with his ninth goal of the season from close range.
However, Killie came very close either side of the goal and they were eventually rewarded for their improvement when Nat Phillips turned Brad Lyons’ cross into his own net in the 75th minute.
Danny Armstrong then slipped Kennedy through as Killie counter-attacked down the right-hand side and the winger’s effort went in off Joe Hart.
Brendan Rodgers had suffered his first domestic cup defeat as Celtic manager at Rugby Park in August and the latest loss on the artificial surface was the first cinch Premiership defeat of his second spell in charge. It leaves Celtic five points ahead of Rangers but with one more game played.
Phillips started after Cameron Carter-Vickers failed to shake off a hamstring concern, but Rodgers otherwise retained the team that began Wednesday’s 4-1 win over Hibernian. That meant starts for Tomoki Iwata, Mikey Johnston and Oh Hyeon-gyu.
Rodgers had told Johnston he had to “do more” following the midweek game. Johnston had a hand in the first goal and could have had an assist but for Oh’s poor finishing, however the winger’s delivery was erratic during his 70 minutes on the park.
Lyons and Liam Donnelly brushed off knocks to start for Killie, who were pinned into their defensive third for the first 10 minutes. O’Riley had two shots stopped, Callum McGregor’s low cross just evaded Johnston and Oh had a header saved.
Celtic continued to create chances. Oh volleyed over after Liam Scales’ ball over the top and the South Korean should have converted Johnston’s low cross before Luis Palma hit the post from a tight angle.
Killie should also have scored from their first chance. Lyons’ cross found Armstrong at the back post but the winger hit the junction of post and bar from six yards.
Celtic were ahead within a minute. McGregor took three players out the game when he turned on to his right foot 20 yards out and forced Will Dennis down to his left with a shot. The goalkeeper could only parry and O’Riley was on hand to convert the rebound.
The action continued as Celtic survived a double chance moments later. Hart got down well to deny Armstrong and Alistair Johnston threw himself at Corrie Ndaba’s follow-up to block what looked a certain goal.
The opening stages of the second half were as one-sided as the first but this time in the home team’s favour. Hart denied Ndaba and Robbie Deas headed just wide amid sustained aerial pressure before the Celtic goalkeeper got down to clutch Armstrong’s curling effort.
O’Riley and Palma threatened after Celtic counter-attacks as the incessant rain got heavier and Kilmarnock appeared to have lost their cutting edge but they took a major lift from the equaliser.
Stuart Findlay had an effort saved after another ball into the box and substitute Marley Watkins scooped just over from a better chance, before Kennedy found the net in the 87th minute.
Celtic brought on Kyogo Furuhashi and Dennis pulled off an excellent stop from the Japan striker 90 seconds after Kennedy’s goal.
The home fans howled with derision when the fourth official signalled there would be eight minutes of stoppage time but Killie comfortably saw them out.