However, Groenewegen - who won the final stage on the Champs Élysées previous year - seemed fresher and more powerful and beat all of his rivals by a sizeable margin.
Other riders chatted as they admired the rolling countryside.
"A lot of people didn't believe in me anymore and thought I wouldn't come back to the same level".
A group of five riders broke early on the 181km run from Brest to Mur-de-Bretagne and when the Quick Step team of stage contender Julian Alaphilippe suddenly accelerated on a plain, the peloton split into three with 100km remaining.
"I just keep trying, our backs are against the wall all the time here", said the 33-year-old, who is in a contract year but is expected to stay with Dimension Data. "I was going from a good position and the legs were good so it was a flawless day". Froome trails race leader Greg Van Avermaet by 1:02. "I was picking wheels and seemed to be in a good position", Cavendish said.
Although Van Avermaet retains yellow for now, it was in truth a dismal day for BMC.
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The Lotto NL-Jumbo rider, who won the final stage to Paris past year, had by his own admission suffered a disappointing start to the 2018 Tour before his show of power to beat Gaviria into second and Peter Sagan into third.
Four-time and defending champion Chris Froome is in 14th place, one minute and five second behind the yellow jersey of van Avermaet.
Since the route was announced in October, this 156.5km stage nine over the cobbles of northern France to Roubaix had been circled as the wildcard which could deliver a jolt to the race, and it did not disappoint.
"It was a very fast ascent and I just held on really", said the Belgian, who finished alongside team leader Richie Porte at three seconds off the pace to pull on the jersey for a third day.
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 pack rides during the seventh stage of the Tour de France cycling race over 231 kilometers (143.5 miles) with start in Fougeres and finish in Chartres, France, France, Friday, July 13, 2018.