He made a break in the last kilometre into Mur de Bretagne (pron: Brit-an-yah) to register his first stage victory at the event since 2013.
Though the stage was billed as one that could change the yellow jersey holder, it is Van Avermaet who remains in the overall lead, having finished 12th, three seconds behind Martin.
"My legs were there all the time, I already wanted to have a go yesterday but it wasn't as steep as today".
Riders twice went up the Mur de Bretagne, or "Wall of Brittany", which the local cycling-crazed Bretons affectionately refer to as their smaller version of the famous Alpe d'Huez. '"With 200 metres to go I saw the gap and I was going to the finish line".
"I was afraid of the crosswinds and maybe it was adrenalin but the legs were there and it all worked out".
Stage eight on Saturday (Sunday NZ time) covers a slightly more challenging terrain over 181km from Dreux to Amiens but again should set up well for sprinters.
"But there's a great feeling in the bus now, laughing and joking and this win is for them, they really looked after me".
"I hit the back wheel of Bardet when there was the movement in the peloton and I couldn't avoid it", the Dutch rider said.
"I had good sensations but nothing outstanding, I was at my maximum".
Outside the AG2R team bus at the Tour de France starting line in Lorient on Wednesday there stood just seven warm-up bikes, with Axel Domont no longer needing his after a 70km per hour fall on the previous day's stage.
Bardet would lose 31 seconds on the stage in a blow to the Frenchman's general classification hopes, while 2017 Giro d'Italia victor Dumoulin lost 53 seconds.
The short climb inside the final kilometre tempted a number of the classics specialists to the fore, including BMC's Greg Van Avermaet in the yellow jersey, but once the road levelled off it was Sagan who still had the most power left in his legs.
There was further fallout too from Tom Dumoulin's time penalty, imposed after Thursday's finish to Mûr-de-Bretagne, when Froome's rival slipstreamed his team auto after suffering a broken wheel five kilometres from the finish.
"He was in effect Romain's bodyguard, the one who would ride beside him and had been due to do so over the first nine stages", said Jurdie, who has been with the team for 12 years. Van Avermaet said. "But everyone kind of enjoyed it - the first day we could really relax".
Friday's 231-kilometer seventh stage will take the cyclists from Fougeres to Chartes.