Paris Saint-Germain failed to bridge the gap between themselves and their Ligue 1 title rivals by slumping to a 2-1 defeat at Guingamp that increases the pressure on Unai Emery.
A quickfire double from Yannis Salibur and Nill de Pauw did the damage midway through the second half, with Edinson Cavani, who had missed a glorious opportunity shortly after half-time, pulling one back with 10 minutes to go to set up a dramatic finale as the champions went in search of an equaliser.
Emery's side looked to have turned a corner with a resilient comeback draw against Nice in their last Ligue 1 fixture, but were lacklustre throughout on Saturday, with their defensive frailties again proving costly.
Guingamp - who move up to fourth with this victory - outclassed PSG in every department.
Despite a late rally following Cavani's 17th league goal of the season and then the striker hitting the crossbar with a looping header in stoppage time before seeing a Jean-Kevin Augustin effort cleared off the line, the visitors failed to find a leveller.
The result leaves PSG, now without a win in their last three Ligue 1 fixtures, planted in third place, giving Monaco and Nice the chance to extend the gap at the top when they face Lyon and Dijon respectively on Sunday.
Fresh from the midweek 3-1 Coupe de la Ligue win over Lille, PSG dominated proceedings early on, though Guingamp's defence remained untested, with Angel Di Maria's wayward long-range strike the highlight of a tepid opening.
Di Maria went close again soon after, but Karl-Johan Johnsson got down well to make a good save from the Argentine's strike while, at the other end, Moustapha Diallo headed wide.
Guingamp's confidence continued to grow, Diallo squandering another close-range chance before Jimmy Briand was denied by Marquinhos' excellent block.
Against the run of play, however, PSG would have had the lead on the half-hour mark had Cavani directed his volley on target after connecting with Di Maria's exquisite cross.
PSG's players were left furious five minutes prior to the interval - Marco Verratti claiming that he had been scythed down in Guingamp's area by Fernando Marcal - but referee Fredy Fautrel, taking charge of his final Ligue 1 match before retirement, correctly dismissed the penalty appeals.
Attempting to inject some energy into PSG's lethargic display, Emery introduced Grzegorz Krychowiak at half-time, but the visitors continued to struggle against Guingamp's disciplined defence.
However, they finally carved open their hosts 53 minutes in - Cavani latching onto Verratti's throughball - but PSG's top-scorer failed to apply the finish from a tight angle.
Guingamp wasted no time in making PSG pay for that miss as, 13 minutes later, Salibur slotted in from close range after capitalising on poor defending from the visitors.
Matters were soon made worse for PSG - De Pauw marking his first Ligue 1 start of the season with a neat finish at the culmination of a sweeping counter-attack.
Emery's side piled forward, but despite having two fantastic chances – Cavani hitting the crossbar with a brilliant header before Johnsson and Marcal combined to clear Augustin's attempt off the line – as Guingamp somehow held firm to claim the points.