Martin, whose other career Tour stage win came in Bagneres-de-Bigorre in 2013, had finished second on this climb when it featured in the 2015 Tour and timed his attack to perfection, launching off the wheel after the Australian had made his own move and riding clear.
The general classification contenders all crossed the line safely in the main group, but BMC's Greg Van Avermaet doubled his slender advantage in yellow to six seconds by winning the bonus sprint 31km from home.
Groenewegen grew up in a cycling environment as his grandfather was a bike manufacturer and his boyhood coach was Jean-Paul van Poppel, victor of nine sprint finishes at the Tour de France. I'll have to get through the Alps and see how I am, see how Froome is and go from there.
"I was afraid of the crosswinds and maybe it was adrenaline but the legs were there and it all worked out".
"It's never good to lose time".
"But there's a great feeling in the bus now, laughing and joking and this win is for them, they really looked after me".
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"It was a boring stage, no wind, no stress just boring, I was talking to everybody", he said. "I had no legs to beat the first two guys".
BMC rider Porte crossed the line three seconds off the pace after the 181km ride from Brest to Mur de Bretagne, with defending champion Chris Froome five seconds further back. "I don't know if he did it on objective but he dropped a lot of people", Sagan said.
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.
"Having already lost 53 seconds on the stage - finishing 46th after a big effort from team-mate Soren Kragh Andersen, in the white jersey, to minimise their losses, he now drops to 19th - 1'23" behind yellow jersey Greg van Avermaet.
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. Van Avermaet said. "But everyone kind of enjoyed it - the first day we could really relax".
On the same long straight uphill three years ago, Martin finished second and he has been hinting for the past few days that he had fireworks planned this year.