Belgium booked a last eight berth to face the Selecao with a last-gasp 3-2 win over Japan in Rostov-on-Don on Monday when Nacer Chadli scored the 94th-minute winner as the Red Devils came from 2-0 down.
Belgium strike back to beat Japan in World Cup epic
Martinez's side will face Brazil in Kazan and he says Belgium must embrace their role as underdogs who have nothing to lose.
"When you play against Brazil, you need to accept they are the best team in the competition, you need to accept your role," said Martinez after beating the Japanese in the last 16.
"They have players who can decide the game on their own like (Philippe) Coutinho and Neymar.
Mertens warns Belgium against complacency
"When we went 2-0 down (against Japan) there was the psychological feeling of 'ok, we have nothing to lose', which freed us up.
"That's like how it will be against Brazil, except we can feel like that from the first second.
"This is the sort of game you dream about as a little boy and we can enjoy it from the first second."
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