Monaco capitalised on Paris St Germain's draw on Saturday night with a 2-0 victory over Marseille that sees them move to just three points behind the pacesetters.
The big-spending principality club were without injured star striker Radamel Falcao but were still too good for their south-coast rivals as goals from Valere Germain and Emmanuel Riviere earned them the win.
This week's loss of Falcao to what looks set to be a season-ending knee injury was a major blow to Monaco's hopes of pipping PSG to the title, but they received an unexpected boost when the leaders were held 1-1 by Guingamp.
Claudio Ranieri's men then ensured they took full advantage of that slip, while inflicting a seventh league defeat of the season on last year's runners-up Marseille.
Monaco were quickly in the box seat and Riviere almost gave them the lead after just two minutes with a stunning 25-yard volley that visiting goalkeeper Steve Mandanda managed to turn behind for a corner.
Andrea Raggi was then denied by Mandanda before Riviere thumped in a right-footed effort that went inches wide as the hosts continued to push for the opener.
Marseille had to wait until the 41st minute for their first real chance, but Florian Thauvin saw his header blocked by Monaco keeper Danijel Subasic and within 60 seconds the home side were leading.
Raggi fed the ball to Germain 30 yards from goal and he lifted the ball past outrushing defender Kassim Abdallah as he raced in the area from the left before rolling his shot just inside the far post.
Monaco kept the pressure up at the start of the second half and James Rodriguez brought another save out of Mandanda before Riviere made it 2-0 in the 58th minute.
Geoffrey Kondogbia curled in a cross from the right of the area which Riviere controlled on the edge of the six-yard box before slamming his shot into the roof of the net.
Marseille pushed forward in the final half-hour in a bid to get back into the game but Subasic ensured Monaco kept their advantage intact with two good saves from Andre-Pierre Gignac and a late stop from Diawara.