Rangers failed to tighten their grip on top spot in the Scottish Premiership after falling to a shock 2-1 defeat at home to Motherwell.
It was the first time since 1997 that Motherwell had triumphed at Ibrox, scoring once in each half to hand initiative back to Celtic in the race for the league title - only for Brendan Rodgers' side to lose at Hearts 24-hours later.
Theo Bair opened the scoring early in the first period, sweeping Jack Vale's precise pick-out beyond the clutches of Jack Butland, before James Tavernier clawed Rangers level from the spot on the hour mark.
Having finally restored parity, Philippe Clement's side looked the more likely to unearth a winner, but for Blair Spittal's rampaging run in the 75th minute, which teed up Dan Casey to head an unlikely decider.
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It marks a first defeat in 12 games in all competitions for Rangers who remain two points clear of their Old Firm rival at the top of the table.
How Rangers' title hopes were damaged
Rangers had fought back valiantly to beat Kilmarnock 2-1 at Rugby Park on Wednesday night, keeping them importantly ahead of the Hoops in a title race that promises to swing in multiple directions before its May conclusion.
It was widely believed that three more points would be garnered against the Steelmen.
But Clement's side were nothing like the team that showed courageous title promise midweek, despite dominating the ball and generating 31 efforts on goal - nine of which hit the target.
The visitors had the first chance when defender Bevis Mugabi headed a Spittal corner over the bar but they did not miss the second. Vale got the better of John Souttar and cut the ball back for Bair to bury beyond a helpless Butland.
The traffic towards the Motherwell goal remained busy but goalkeeper Liam Kelly was equal to nearly all, barring a Tavernier penalty, which was dispatched expertly after Stephen O'Donnell was guilty of fouling substitute Fabio Silva in the box and VAR intervened to overturn referee Alan Muir's original decision.
Rangers threw everything forward in the final stages, but it was Spittal who sprung the trap to find Casey's late run, and he beat Butland at his near post.
Clement: Poor start cost us
Rangers boss Philippe Clement:
"We didn't start the game well, in a way that is not normal for our team. We pushed to score more goals than them, we had more chances, more possession, and it's a game that normally we would win.
"One player was kicked off the pitch, without a foul or a card. That's what I saw today. Last week we were much more efficient, today we created much more but only score one.
"I cannot say about the mentality of my team. They pushed, they gave everything. You cannot ask more. But maybe all we ask is that we came a little bit too much in a rush in key moments, and that's a work in progress."
Kettlewell: Result long overdue
Motherwell boss Stuart Kettlewell:
"The start was a huge aspect. I've been saying it for the last year or so, we've been so competitive against Rangers and Celtic.
"We believe in what we're trying to do. We showed lots of quality and carried a threat throughout the game. It's nice to see our goalkeeper perform so well. To a man, I thought the players were outstanding."
What's next?
Rangers' attention turns to the Europa League with a trip to Libson to face Benfica in the first leg of their last-16 tie on Thursday night. Kick-off 8pm.
They then return to domestic action on March 10 with a Scottish Cup quarter-final away to Hibs. Kick-off 5.30pm.
Motherwell's next match is at home to Aberdeen in the Scottish Premiership on March 16. Kick-off 3pm.