"It's hated. That kind of climb favours the punchy, pure climbers. Egan for one, and Yatesy (Adam Yates), Riche Porte and Nairo Quintana in other teams," the Welshman told reporters after Wednesday's fifth stage.
Peter Sagan edges Wout van Aert and Matteo Trentin to win Stage 5
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.
But that is no indication of form, Thomas said earlier this week.
"I think we will know more tomorrow," he said.
"We will see how it is. I'll just go and try to ride the best I can and see where I'm at on top."
Full highlights of a Sagan masterclass in Stage 5
"I raced there a couple of years ago when I lost the yellow jersey so not the best memories," he added.
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.
Geraint Thomas - 'It feels totally different being defending champion, but it's a nice boost'
Thomas is seventh overall, 45 seconds behind leader Julian Alaphilippe of France.