The Barcelona forward is preparing to face club team-mates Neymar and Dani Alves, however, Sanchez made his concerns public about how the game would be refereed - with the implications being favouritism to the hosts may be a factor.
Brazil were awarded a soft penalty in the tournament opener against Croatia with the score at 1-1 heading towards the final 20 minutes.
English referee Howard Webb has been appointed to the fixture in Belo Horizonte on Saturday, but prior to the distribution of refs for the last 16, Sanchez said he doubted whether Chile would get a fair go.
"I think Brazil is the team with the best defence," Sanchez said.
"They've got great quality in their squad, and I'm really happy we are going to play against Brazil and Neymar.
"My worry is the referee, but we'll see."
Despite his concerns, Sanchez did concede he was thrilled to play the tournament favourites and his Barca team-mates.
"I'm very happy playing against Brazil, I am delighted to play against some of my club team-mates like Neymar and Dani Alves, and I will enjoy this game a lot," he said.
"It is something that fulfils me as a player, and as a team too, so we can demonstrate that we can hold our own with the best.
"Of course I respect them, they have a great team and great players."
Mauricio Isla said Chile are susceptible to the long ball, adding that Brazil are stronger now than four years ago in South Africa, where the South American nation won 3-0 in their last-16 match-up.
"I think this will be a very different game to the one we had against them in South Africa, I remember that in that game, they got the opener from an aerial ball," midfielder Isla said.
"That kind of play, has always been tough for us."
Isla added, alluding to past Chile players: "It would be nice to have an Elias Figueroa or a Chano Garrido in our defence, because we are short.
"But I also believe that this Brazil plays much better than the one in 2010.
"They have got great defenders that can play as midfielders.
"I think they have the best central defenders in the world, and it's going to be a different game."
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