Rene Girard's Lille maintained their remarkable defensive record in defeating fellow high-fliers Marseille 1-0 at Stade Pierre-Mauroy.
An 11th consecutive clean sheet for Vincent Enyeama gained added value as Nolan Roux grabbed an injury-time winner to give the hosts all three points.
Lille now sit just one point behind PSG, whose game in hand comes at Evian on Wednesday, while Monaco are a further point back after a 3-0 triumph at Nice.
Goals from James Rodriguez and Emmanuel Riviere gave Claudio Ranieri's men control inside 23 minutes.
Nice were unable to respond and Monaco substitute Lucas Ocampos rounded off the scoring with a minute remaining.
Elsewhere, Nantes bounced back from successive defeats and climbed to fifth with a 2-1 home win over lowly Valenciennes.
Jose Saez put the visitors ahead in the 52nd minute with his second goal of the season.
However, Alejandro Bedoya soon levelled and Nantes snatched victory through Fernando Aristeguieta with a minute of normal time remaining.
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