Brad Lyons grabbed the decisive goal as Kilmarnock stunned Rangers to secure a 1-0 victory on the opening day of the Scottish Premiership season.
Rangers, who handed competitive debuts to five players, produced a disjointed performance at Rugby Park, creating few really clear-cut chances.
They were eventually punished when they failed to clear a long throw midway through the second half and Lyons pounced, volleying past Jack Butland as Rangers suffered their first opening-day top-flight defeat since a loss at Hearts in 1998.
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Even at such an early stage of the season, the defeat is a setback for Michael Beale's side, who fall three points behind Celtic following Brendan Rodgers' side's 4-2 win over Ross County earlier on Saturday.
How Kilmarnock stunned Beale's side...
Rangers had a new look about them with Beale handing competitive debuts to five players, and it showed as they looked slightly disjointed against an organised Kilmarnock.
Abdallah Sima, the striker on loan at Ibrox from Brighton, produced a clever back-heel flick in an attempt to catch goalkeeper Will Dennis off guard, but the on-loan Bournemouth man made a comfortable save.
Clear-cut chances were at a premium as James Tavernier's inswinging corner bobbled inside the Kilmarnock penalty area, but Dennis did well to gather the ball low down.
Rangers started to have more joy as the half wore on. Sima's cross was cleared only as far as Cyriel Dessers, but the forward's shot was well saved by Dennis at his near post.
Dennis was again called into action just before the break, this time tipping John Lundstram's powerful drive over the crossbar after the Rangers midfielder was teed up by Dessers.
After the break, Rangers continued to ask a few questions of Kilmarnock as Kieran Dowell's long-range effort deflected over the crossbar. From the resultant corner, John Souttar's header dropped inches wide of the far post.
Beale turned to his bench in search of an answer as Todd Cantwell and new Brazilian signing Danilo came on, but it was Kilmarnock who took the lead as a long throw caused chaos in the Rangers defence.
Rangers failed to clear their lines and Lyons pounced, volleying past Butland and into the far corner to spark wild celebrations at Rugby Park.
The goal breathed further life into Kilmarnock as Kyle Magennis forced Butland into a save at his near post before another fast counter-attack from the hosts ended with Daniel Armstrong curling wide.
As the game wore on Kilmarnock eventually dropped deeper with Rangers pushing hard for a point, but the hosts held firm as the celebrations sparked up again for the Killie fans and players.
Boyd: Kilmarnock were well worthy of the three points
Sky Sports' Kris Boyd:
"Rangers have to defend the goal better. You knew coming here it was going to be a very different game for Rangers.
"Kilmarnock were well worthy of the three points here today. You have to pay them respect because in terms of the set-up, yes, but they then did carry that threat.
"It was a real good team performance and it is not a case of discrediting Kilmarnock, they gave as good as they got and deserved the three points.
"And Beale is now under pressure, you know what happens when you lose a game for Rangers, you saw the fans at the end, they were not happy."
Beale: It's a very poor opening day
Rangers manager Michael Beale:
"Kilmarnock defended extremely well. They had a game plan. They were dangerous on the counter all day. They were comfortable in the game.
"It's a very poor opening day for us. We need to move forward very quickly. There's 37 games to go, let's not get carried away.
"The excitement over the summer is going to be tempered. We have to show the fans and excite them and they have to get behind us, it works both ways.
"I expect us with the quality we have to execute in the moments we had. If we don't, I expect a hard-fought away draw rather than coughing up a really poor goal."
McInnes: We'll get better
Kilmarnock manager Derek McInnes:
"You have got to do so much right to beat a team with Rangers' quality.
"We spoke at half-time about striking the first blow against Rangers to have something to hold on to and to plant that seed of doubt. We knew, if we did that, we'd have a real good chance.
"We had to show a different version of ourselves against the Old Firm this season. Rangers will get better, but I think we'll get better as well.
"Our level of performance had to be really good today, but we delivered that. It's a really strong start."
What's next?
Rangers' next Scottish Premiership match is at home to Livingston on August 12, kick-off 3pm.
Kilmarnock are away to Hearts the following day at 3pm.