Just behind this victorious pair the struggle between the contenders to win the Tour de France itself played out when defending champion Geraint Thomas suggested that the doubters who had called him a one-hit wonder after his 2018 win with Sky might be wrong.
The 175.5km stage from Saint-Die-des-Vosges came down to a sprint finish as the quickmen kept pace with the peloton over the late climb of Cinq Chateaux.
He was fourth over the line in the first mountain stage of the competition.
Twice champion Vincenzo Nibali, who was waiting for the first mountain stage to assess his form after finishing second overall in a demanding Giro d'Italia in May, lost 51 seconds to Thomas, whom he now trails by 1:07 overall.
Mitchelton-Scott's Trentin launched his sprint first but Sagan had the power to come around.
Defending champion Geraint Thomas gained time on his rivals in a brutal finish in the sixth stage of the Tour de France as several top guns cracked in the lung-busting ascent to La Planche des Belles Filles on Thursday.More news: Interception of vessel with 7 tons of cocaine in the US
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Dan Martin and Nicholas Roche both finished in the leading group.
As Dylan Teuns won stage six from the breakaway and young Italian Giulio Ciccone snatched the yellow jersey off the shoulders of Julian Alaphilippe, it was the sight of Thomas defying his own predictions and bursting clear of the main contenders at the last which caught the eye.
"An initial observation following the stage indicated he had some rib pain and other contusions, but he would be able to line up on stage five", said CCC Team Chief Medical Officer Dr Max Testa.
"Now it feels unusual to be here with the yellow jersey myself", added Ciccone, who hails from the Abbruzzo region. The stage ended up as a battle not only for the stage and the overall lead but for the psychological control of the race. "I thought it would be more of a solid day".
"It feels odd, to have the yellow jersey on my back feels so unusual." said Ciccone. You can't really tell a lot from the last few days - obviously Alaphilippe is flying up those type of climbs.
Friday's seventh stage, the longest of the 2019 Tour, returns to flatter terrain as the peloton races from Belfort to Chalon-sur-Saone.