Five years after his first stage win and the last Irish victory on the Tour, at Bagneres-du-Bigorres in the Pyrenees, Martin took glory on the shorter but equally steep slopes of the aptly named Mur de Bretagne, or "Wall of Brittany".
"There's no reason I can't look at it [the Mûr de Bretagne], even though I'm contending for GC [the general classification]", Martin said earlier this week.
"I'd like to thank Greg (Van Avermaet) because he did me a favour when he went for it, I don't know if he did it on goal but he dropped a lot of people", said Sagan, who has tightened his grip on the points classification's green jersey.
Dylan Groenewegen hasn't been in the right place to show his sprinting legs but this resembles the stage of Paris-Nice that he won.
What time does the stage start? "I thought the corner was closer to the finish than it really was".
Having finished sixth on the previous day's uphill finish and remembering back to his second place behind another Ag2r rider, Alexis Vuillermoz, when the Tour was last on the Mur-de-Bretagne back in 2015, Martin gambled on an early attack yesterday.
"The other day I was with a youth cycling coach and before races parents have been giving Ventolin to children who aren't sick", the Frenchman said. "I was picking wheels and seemed to be a good position".
While Australian Richie Porte, Italian Vincenzo Nibali and Britain's Geraint Thomas all crossed the line three seconds off the pace after the 181-km ride from Brest, defending champion Chris Froome was five seconds further back.
AG2R's Romain Bardet cracked in the heat of the final ascent and lost 31secs, while fellow contender Tom Dumoulin (Sunweb), the 2017 Giro d'Italia champion, lost almost a minute after suffering a mechanical problem.
Dumoulin and his Sunweb team compounded the mechanical mishap by getting a 20-second penalty when Dumoulin stayed too close to the team vehicle while trying to catch the pack, thereby benefiting from its draft that pulled him along.
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.
The slightly-built Bardet's AG2R team will have to race 17 stages of the 2018 Tour a man down after an over-curious roadside fan stepped into the path of the onrushing peloton to take a photo.