Ranieri's side lost 1-0 at seventh-placed Reims in the Coupe de la Ligue in October, a month after being held to a 1-1 draw at Stade Auguste-Delaune II in the French top flight.
Monaco get a third opportunity to see off Hubert Fournier's side on Friday and the Italian knows they will have to be at their best at Stade Louis II in order to secure all three points.
He said: "Reims are the third-best team behind us and PSG.
"They are a 'champagne' team and we have already played them twice without beating them.
"So now we must take them very seriously and finally do so.
"Tactically, they are very compact and organised in defence, while they also have good attacking options and pace."
Monaco are five points behind leaders Paris Saint-Germain and seven points better off than third-placed Lille.
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