Paris Saint-Germain showed evidence of their vulnerability early in the reign of new head coach Unai Emery on Sunday, the Ligue 1 champions losing 3-1 at Monaco.
PSG would have gone two points clear at the top of the table with a win, but instead they struggled to cope with the intensity shown by the home side and fell behind on 13 minutes when Joao Moutinho slammed home a half-volley from just inside the box.
And a brittle defence was undone again just before the break as David Luiz pulled Jemerson down in the box and Fabinho converted the resulting spot-kick.
Edinson Cavani did head PSG back into the contest shortly after the hour-mark, but as they mounted some concerted late pressure, Serge Aurier inadvertently diverted Djibril Sidibe's cross into his own net to make the game safe for Monaco.
Leonardo Jardim's side impressed with both their work rate and pace going forward and also showed plenty of character as they dug deep to secure their first home win over the Parisians since 2009.
The result extended the principality side's unbeaten start to the campaign and moved them a point above the reigning champions in the standings.
PSG looked the more likely during the opening exchanges and yet it was Monaco who took the lead with what was their first meaningful attack of the game.
Andrea Raggi sent Sidibe away down the right and his low cross was swept home from just inside the box by Moutinho.
And the home side should have doubled their lead moments later when Sidibe was again allowed to deliver from the right flank, but this time Valere Germain fluffed his lines from close range.
Cavani was then similarly wasteful in front of goal for the visitors on 21 minutes, snatching at his shot when played in by Aurier and firing wildly wide.
Seemingly unsettled by the intensity of their pressing, PSG struggled to impose themselves upon Monaco, with Aurier's forays down the right providing their only serious attacking threat.
The hosts, in contrast, looked menacing on the break and again went close to doubling their advantage nine minutes before the break when Bernardo Silva fired narrowly wide of Kevin Trapp's right post.
They were not to be denied in first-half stoppage time, however, as David Luiz gifted them a penalty by needlessly hauling Jemerson to the ground.
The former Chelsea man earned a yellow card for his troubles before Fabinho rifled home from the spot to make it 2-0.
PSG started the second half on the front foot and came close to halving the arrears on 54 minutes when Lucas Moura got the better of Benjamin Mendy down the right and whipped in a low cross only for Cavani to completely miss the ball as he attempted an audacious near-post flick.
The Uruguayan was far more clinical with his next sight of goal, however, steering a fine header past Danijel Subasic after being teed up by the lively Aurier.
Cavani almost levelled things up moments later, just failing to slide the ball home from close range but just as PSG seemed poised to launch a late charge, Monaco bagged their third.
Silva released Sidibe and as he squared the ball across the box, it struck the covering Aurier and cannoned into the back of the net.
