Joining Porte in those time gains was Irish stage victor Daniel Martin (UAE), Spaniards Alejandro Valverde (Movistar) and Mikel Landa (Movistar), Briton Adam Yates (Mitchelton-Scott), Welshman Geraint Thomas (Sky), Colombian Nairo Quintana (Movistar), Italian Vincenzo Nibali (Bahrain-Merida), and Dane Jacob Fuglsang (Astana).
The likes of Vincenzo Nibali, Richie Porte, Geraint Thomas, Nairo Quintana, Mikel Landa and even Peter Sagan were part of the group - although defending champion Chris Froome (Team Sky) found himself on the other side of a slight split to concede five seconds to many of his GC rivals, while Colombian Rigoberto Uran (EF Education First-Drapac) was a further three seconds in arrears.
Yellow jersey leader Greg Van Avermaet extended his lead in the overall rankings to six seconds by winning the bonus point with 30km to go. The closing kilometres of the stage were unusually free of crashes meaning there were no major changes in the general classification.
Tom Dumoulin, victor of the 2017 Giro D'Italia, suffered badly following a mechanical issue with only 5km to go, dropping 40 seconds on the leaders.
He was then handed a 20-second penalty by the race jury for staying too long in his team car's slipstream.
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".
"But there's a great feeling in the bus now, laughing and joking and this win is for them, they really looked after me".
"So I set off, kept my head down, kept going and my legs felt good".
"It was hard because there was a headwind, so going solo was risky".
"Maybe they should just have the stages at less than 200km in Grand Tours, it would be more exciting for everyone".
The 231km stage from Fougeres to Chartres saw multiple breakaway attempts, but on a flat route with occasional crosswinds, none seemed likely to stick - not least because the majority were undertaken by lone riders.
This year the stage will be expected to finish in another bunch sprint, but the potential for strong winds to form could further impact the general classification if teams are not switched on.