Since the Planche des Belles Filles featured for the first time on the Tour in 2012, the rider in yellow after that stage went on to win the race - Bradley Wiggins in 2012, Vincenzo Nibali in 2014 and Chris Froome in 2017.
Stage six sees the race stay in the Vosges and features the first summit finish of the 2019 Tour. "It's the dream of any person who loves cycling", said the 27-year-old Ciccone, who snatched the jersey thanks to 14 bonus seconds he grabbed at top of the Col de Chevreres and by finishing second in the stage.
"I think we will know more tomorrow", he said.
Sagan's win will be popular with armchair fans as the charismatic former triple world champion, and six-time green sprint jersey victor had been frustrated in two near misses so far.
"I try every day, and then one day you get it", said the burly Sagan. 'I thought the steep climbs weren't my cup of tea.
The Slovakian edged rising star Wout Van Aert of Belgium and Italy's Matteo Trentin to win the stage in 4:02:33.
Thomas already lost five seconds to Colombian Bernal when he found himself at the wrong end of a split in the third stage on Monday. "I'm 47 points ahead in the race for the green jersey now".
Thomas, on the back foot after being left behind by team-mate and Ineos co-leader Egan Bernal at the finish of stage three in Epernay, showed that he is still the boss.
Thomas was typically drole when asked what he expected as the Tour headed off in the morning quipping: "I'm expecting the worst and hoping for the best".
Welshman Thomas looked relaxed after the race.
What Thomas describes as "the big day" is a seven mountain slog culminating in a summit finish at La Planche des Belles Filles where four-time Tour de France victor Chris Froome enjoyed a breakout win in 2012.