Monaco missed the chance to move top of Ligue 1 after suffering a shock 2-1 home defeat to Valenciennes.
An Eric Abidal own goal and David Ducourtioux's first strike of the season proved to be enough to hand Daniel Sanchez's men only their third win of the season.
James Rodriguez pulled one back late on for the hosts after Radamel Falcao had missed a penalty, but it was too little too late.
Claudio Ranieri's men remain second, but have played one more game than their title rivals Paris St Germain and Lille, who meet on Sunday.
And they will go into the winter break with a sense of frustration following their first defeat at home this season.
Monaco dominated the early exchanges with Mounir Obbadi going close twice inside the opening 25 minutes.
First the former Troyes midfielder saw a shot blocked by Ducourtioux after Layvin Kurzawa picked him out before he stole possession on the edge of the Valenciennes area before forcing a save out of Nicolas Penneteau.
However, the Stade Louis II was stunned into silence with half an hour played when the visitors took an unlikely lead in fortunate circumstances.
Youngster Arthur Masuaku got beyond his man on the left before delivering a hopeful cross which cannoned in off Abidal's shin after goalkeeper Daniel Subasic had missed it.
Monaco thought they had replied when Rodriguez superbly danced his way into the area and, after Anthony Martial saw his shot blocked, Kurzawa bundled the ball over the line, but the assistant raised his flag to deny Ranieri's men.
Valenciennes continued to live dangerously and were thankful of a goal-line clearance from Ducourtioux when fellow defender Gary Kagelmacher directed a header goalwards following a Joao Moutinho corner.
The hosts continued to press for an equaliser and could have had a penalty when Martial took a tumble inside the area, but referee Olivier Thual was unmoved.
Moutinho then grazed a post with a superb free-kick as the visitors took an unexpected lead into the break.
In the second half Moutinho went agonisingly close again with a curling 25-yard free-kick before Martial cut in from the right and tested Penneteau with a stinging left-footed effort.
The hosts rode their luck when, having thrown bodies forward, Mathieu Dossevi went close on the counter-attack, his curling effort palmed away by a diving Daniel Subasic.
Monaco chose to ignore that warning and it proved costly as, once again on the counter, Ducourtioux was sent clear and he kept his calm to roll the ball beyond Subasic to send the visitors into a 2-0 lead.
With time running out the hosts won a penalty when Falcao went down in the area, but the Colombian stepped up and saw his weak effort saved by Penneteau.
Rodriguez to found the net with his head from the resulting corner, though, to give Monaco a lifeline.
However, despite Monaco having a late Rodriguez strike ruled out for offside, VA held out to claim a notable win.