For half an hour the Parc des Princes purred with the certainty of old. Marquinhos had risen to nod them ahead, the champions looked in command, and the natural order of Ligue 1 seemed intact. Then AS Monaco remembered they had arrived in Paris as the division's only team with a perfect record, and they left with it burnished still brighter.

Two second-half goals — a header from Flávio Nazinho and a piece of pure improvisation from Stanis Idumbo — turned this Friday-night showpiece on its head. Paris Saint-Germain, once again, were undone by their own fragility, beaten 2-1 on their own turf and left to stew in the lower reaches of a table they are accustomed to owning.

It was a result heavy with early-season meaning. Monaco climbed to the top on nine points from three straight wins; PSG sank to twelfth, marooned on two, their season already carrying the whiff of crisis before the leaves have turned. The margin was one goal. The gulf, on the night, felt considerably wider.

A false dawn

The opening exchanges belonged to the hosts, and when the breakthrough came it wore a familiar name. A corner swung in, Khvicha Kvaratskhelia was alert to the chaos, and there was Marquinhos on 31 minutes to stab home from very close range into the high centre of the goal. The captain's celebration was all relief, the roar around him all release.

For those first 45 minutes it looked like the tonic PSG craved. Kvaratskhelia's delivery had unlocked the visitors, the lead was deserved, and the anxiety that has trailed this side through their stuttering start seemed briefly to lift. A goal to the good, a full house behind them, the champions appeared to be steadying.

But a single goal has rarely felt so precarious. Monaco did not fold at the interval; they regrouped. The lead that flattered PSG in the first half would look painfully thin the moment their visitors decided to press their claim, and that moment was not long in coming.

The tide turns

Thirteen minutes into the second half, the equaliser arrived with brutal simplicity. Jordan Teze worked the space and delivered, and Flávio Nazinho met it with his head, guiding the ball from very close range into the bottom-left corner. The Parc fell quiet, the sense of inevitability draining from the home support.

The goal changed the temperature of the contest. Monaco, unbeaten and unbowed, smelt something more than a point, and PSG, so brittle in these situations, began to wobble. The lead had gone in an instant, and with it went the composure that a nervous side can never quite reclaim once it slips.

Fourteen minutes later came the moment of the match, and it was a thing of nerve and quality. Aleksandr Golovin fed Idumbo out on the left, in a position that promised little. What followed was audacious: a left-footed shot from a difficult angle that arced beyond reach into the top-left corner. It was the strike of a player who backed himself when the percentages said otherwise, and it left PSG chasing a game they had begun in command.

Paris in the mire

The numbers make grim reading in the capital. Three matches gone and Paris Saint-Germain have won none of them, two draws and now a defeat leaving them twelfth with two points, six goals conceded, and a defence that has offered opponents too many invitations. For a club that measures its seasons in trophies, a start this feeble is a genuine alarm.

This was not a night undone by fine margins so much as by recurring weakness. Twice PSG were opened up by crosses into their own box, twice they failed to respond, and the lead handed to them by Marquinhos was squandered with a passivity that will trouble them long after the whistle. The talent in the squad is not in question; the resolve is.

The Parc des Princes has seen enough coronations to know what a stumble looks like, and this was unmistakably one. The champions do not panic easily, but they cannot pretend twelfth place and a solitary win column reading zero is anything other than a warning. The reckoning, if it is coming, has begun early.

Monaco on top

For AS Monaco, the reward is the view from the summit. Three games, three wins, nine points, and now a scalp taken in the grandest arena the French game can offer. Five goals scored, only one conceded, and a swagger that comes from beating the country's biggest club in its own house. This was a statement, delivered with the calm of a side that believes in itself.

There was something instructive in how they won it, too. Beaten to the first goal, they did not flinch; they trusted their pattern, found their equaliser through Nazinho, and then unlocked the game with the sort of individual brilliance from Idumbo that separates the ambitious from the pretenders. Golovin's craft ran through both second-half strikes, the veteran's fingerprints all over the comeback.

The season is young and titles are not won in September, but Monaco have served notice that the top of Ligue 1 will not be conceded meekly this year. They travel home from Paris with maximum points and the position to match. On this evidence, they intend to keep it. The champions, for now, must watch them from below.