And the prospect of seeing the two born competitors face off on a football pitch has Ancelotti intrigued to see how the battle between the two stars and their countries will pan out.
"I have seen these two boys sweating over an eight-a-side game many times during my two years at Madrid. All the great players are competitors," Ancelotti wrote in his column for The Telegraph.
"They all want to come out on top even when the only thing at stake is to be able to walk into the training-ground canteen as the winner.
"So when it comes to the game in Lyon, just think what it will mean to these two lads to do it for their countries."
Ancelotti said he saw how much playing for their national teams meant for both players and even recalled talking to Bale about Wales' chances at Euro 2016 before he left Madrid at the end of the 2014-15 season.
"Gareth always told me that Euro 2016 was a big tournament for Wales," the Italian continued.
"That if they could qualify then who knows what could happen?"
Ancelotti said Portugal were lucky to have not just a talented player in Ronaldo, but a fierce leader as well - qualities which the Italian felt the attacker showed during his country's penalty shoot-out win against Poland.
Ancelotti said outside of the star duo, Portugal had the better team on paper but said Wales' conviction could prove telling.
"For this semi-final, it is clear to me that Portugal have the better players – Cristiano, Nani, Renato Sanches, Joao Moutinho," Ancelotti said.
"Wales have Gareth but they are without Aaron Ramsey. Wales have a strong team though, and that can carry you through a lot of difficult moments.
"A lot will rest on Gareth inspiring his team-mates, but when he says that he believes in them all, you know that he is telling the truth."