"I was a bit nervous because of the headwind, I didn't think it was going to happen", Martin, 31, said.
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.
"I don't know if he did it on objective or if he wanted to drop everybody but I have to say thanks", Sagan said.
"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 was afraid of the crosswinds and maybe it was adrenaline but the legs were there and it all worked out".
"The first days were not good enough, the legs were not good".
"It was quite long - 230k".
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.
After a few early attacks failed, Yoann Offredo launched a solo effort 35 kilometers in.
The Lotto NL-Jumbo rider, who won the final stage to Paris a year ago, 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.
Laurent Pichon, another French rider with Fortuneo-Samsic, then also got away alone for a spell.
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.
The overall favorites should be tested again in Roubaix, before heading down to the Alps next week. But, in the end, it came down to a bunch sprint as expected.
The Dutch cyclist, who rides for Team Lotto NL-Jumbo, clocked almost six hours over the 231-kilometer (143.5-mile) trek from Fougeres, home to the best-preserved and largest medieval fortress in Europe, to Chartres, site of a vast cathedral known for its stained-glass windows.
"Yellow jersey wearer Van Avermaet described the climax as 'an awesome finish".