On a rolling, twisting 204km stage, which demanded technical skills throughout and canny tactics at the finish, Sagan's ability came to the fore.
However, Groenewegen - who won the final stage on the Champs Élysées a year ago - seemed fresher and more powerful and beat all of his rivals by a sizeable margin.
"I positioned myself in Kristoff's wheel and with two hundred metres to go I thought: this is the moment".
Mitchelton-Scott's Adam Yates and Sky's Chris Froome remain 13th and 14th, with their deficit to yellow growing to 65 seconds.
While his team once again showed cracks when finishing 15th of 22 squads in the stage three team time trial in Cholet last Monday, Martin's victory yesterday sees him move into 21st place overall, a minute and 27 seconds behind current race leader Greg Van Avermaet of BMC. Sagan also thanked his team colleagues and his rival Greg van Avermaet, for starting his sprint too soon and thus helping Sagan and Colbrelli pick it up despite the mild hill in the final phase.
"Gilbert attacked a bit early, trying to drop everybody".
With Team Sunweb's Michael Matthews forced to abandon the race in the morning, a sixth green jersey in seven years already looks like being Sagan's to lose. "I don't know if he did it on goal but he dropped a lot of people", Sagan said.
Peter Sagan claimed his second win in the 2018 Tour de France on the fifth stage, moving level in his duel with Colombian rider Fernando Gaviria. The French rider with the Wanty-Groupe Gobert team established an advantage of more than eight minutes - the biggest breakaway lead in this year's Tour - before being caught by the pack with 90 kilometers to go.
"If tomorrow is like that and all goes well on the cobbles on Sunday, it would be great to go into the rest day with the yellow", said Van Avermaet, who will be helping GC contender Richie Porte to keep out of trouble on the Roubaix stage.
Dumoulin then ran into mechanical issues that could badly hurt his Tour de France hopes. The four-time Tour victor, aiming for a fourth successive Grand Tour victory, went straight on at a bend entering the eighth cobbled sector but recovered to finish in the main group.
Friday's flat 231-kilometre leg from Fougeres to Chartes is the longest stage of this Tour.