An early goal by Guingamp's Jeremy Pied was enough to hand champions Paris St Germain their first loss of the French Ligue 1 season, leaving Olympique de Marseille still on top despite their own defeat by Monaco on Sunday.
In the first of the day's twin 1-0 upsets -- which saw the top two felled by lower-ranked rivals -- Pied's diving header in the 11th minute secured a home victory for the unheralded Breton side.
It was PSG's second defeat of the week, following their 3-1 Champions League loss in Barcelona on Wednesday.
Despite strong early possession and a searing shot over the bar by Edinson Cavani in the second minute, the Parisians often appeared jaded against an increasingly combative Guingamp, fresh from consecutive victories over Reims and Caen.
Guingamp's break came when a Younousse Sankhare header steered a free kick to Pied, whose deadly lunge from near the penalty spot left a beaten Salvatore Sirigu fuming.
PSG striker Zlatan Ibrahimovic was the most profligate, miscontrolling incoming passes and sending a free kick into the wall. Team mate Marco Verratti's 19th-minute strike from outside the box found Jonas Lossl's waiting gloves.
The 13th-placed Guingamp also came close to further goals, with Claudio Beauvue often looking dangerous and toying with David Luiz in the PSG box before putting the ball wide in the 22nd minute.
PSG's efforts became more frantic in the second half but ultimately no more effective. Yohan Cabaye earned his latest yellow card for a tardy tackle on Lionel Mathis.
With the pace intensifying, a fine Lucas Moura strike was deflected wide in the 77th and a Cavani cross could only find empty space. Gregory van der Wiel's well-placed centre brought Ibrahimovic close to redemption in the closing minutes, but Guingamp's Danish keeper was up to the challenge.
Guingamp's prowess handed OM a chance to widen their one-point lead over second-placed PSG but that too was squandered.
Monaco's coup de grace came in the 67th minute when Bernardo Silva netted following an Anthony Martial centre and Yannick Ferreira Carrasco feint.
The improving principality side, now up to sixth having let their best players leave following last term's second-place finish, almost doubled their lead via Andrea Raggi's head four minutes later.
Andre-Pierre Gignac and Mario Lemina had earlier come close to scoring for Marseille, whose 38 points represent their highest return at this stage of a season since 1998-9.
Image: PSG's David Luiz (right) is challenged by Guingamp players during their Ligue 1 match on Sunday
Photograph: http://www.psg.fr/Twitter
