Paris Saint-Germain lost more ground on Ligue 1 leaders Nice as they were beaten 2-1 at Guingamp.
Second-half goals from Yannis Salibur and Nill de Pauw stunned Unai Emery's team and, despite an 80th-minute Edinson Cavani effort, the reigning champions were left four points adrift of the summit.
Victory for Antoine Koumbare's team, only their second in the previous seven, sees them move up to the dizzy heights of fourth, seven points behind third-placed PSG.
Emery's team started on the front foot as only a well-timed intervention from Guingamp defender Christopher Kerbrat prevented the in-form Cavani from adding to his season tally of 22.
Not to be outdone the hosts responded, but Jimmy Briand failed to find the target after being out-muscled by the PSG defence before the same player burst clear on goal only to see his goalbound effort superbly blocked by the returning Marquinhos.
Then, just before the hour mark, Emery's team wasted the best chance of the half so far after another perfectly weighted Angel Di Maria pass picked out Cavani but the Uruguayan's volley ended up on the wrong side of the upright.
The champions continued to press and had claims for a penalty turned down on 37 minutes when Marcal collided with Marco Verratti inside the area as the end to a goalless first half drew near.
Emery brought on Grzegorz Krychowiak for Thiago Motta in what appeared to be an attack-minded change and the visitors could easily have broken the deadlock five minutes after the restart.
Cavani and Di Maria linked up superbly again but, after the latter teed up Ligue 1's top scorer, the Uruguayan saw his effort brilliantly kept out by Karl-Johan Johnsson.
It was proving to be a far from convincing performance from PSG, though, and Guingamp almost took full advantage but Moustapha Diallo was unable to steer a header on target from Yannis Salibur's corner.
Then, midway through the second half, Guingamp stunned their visitors when Lucas Deaux picked out Salibur to steer a clinical finish beyond Alphonse Areola and into the bottom corner.
And, not content with just the one goal, EAG doubled their lead just four minutes later.
PSG failed to make the most of their corner and the hosts broke effectively with Salibur and Briand combining well to pick out De Pauw, who confidently slotted beyond Areola, much to the horror of the on-looking Parisians at the Stade Municipal.
Emery's team were not done and, in what was proving to be a thrilling second half, they hit back immediately.
It was all about Layvin Kurzawa's assist as his pinpoint pass fell at the feet of Cavani who finished with aplomb for his 17th league goal of the season.
And they almost salvaged a point deep into stoppage time but the unfortunate Cavani cracked a looping header against the bar before Jean-Kevin Augustin had a very late effort cleared off the line.