Lens denied Monaco second place in the table and automatic qualification for the Champions League group stage when they scored an injury-time equaliser to draw 2-2 on the final night of last season. Philippe Clement's Monaco side - who had won nine league games in a row before that match - were then dumped out of Europe's elite club competition by PSV Eindhoven in the third qualifying round earlier this month.
They had taken four points out of a possible six from their first two Ligue 1 games of this campaign, but a nightmare Saturday afternoon began when Lois Openda tapped in at the back post in the seventh minute. Belgian international Openda joined Lens in the close season having previously played under Clement at Club Brugge.
Lens, who had also won in Monaco in each of the last two campaigns, then doubled their lead when Deiver Machado pounced on an error by Vanderson to run through and score. Benoit Badiashile headed Monaco back into the game just before the break, but Lens restored their two-goal advantage 10 minutes into the second half as Seko Fofana converted a penalty after Florian Sotoca had been brought down by Breel Embolo.
Brazilian full-back Vanderson was then sent off for a second yellow card to leave Monaco to play out the final 18 minutes a man short at the Stade Louis II. Their misery was compounded as substitute Wesley Said blasted in a fourth goal for Lens in the 78th minute. Lens go provisionally top of the fledlging standings with seven points, although Paris Saint-Germain and Lyon are just a point behind.
Lyon won 4-1 at home to Troyes on Friday, while reigning champions PSG can reclaim the lead as they go to Lille on Sunday. Saturday's late Ligue 1 game sees Marseille play host to Nantes.