The two countries will meet in Durban on June 25 in the last Group G match of the tournament which runs from June 11 to July 11.
"It will be a special game because I am Brazilian, a totally different game from all the others. I chose to play for Portugal and I will defend that shirt until the end. When I made the decision I knew these matches could happen," the midfielder told the A Bola sports daily.
"If I score, no Brazilian can see that as a betrayal against the country in which I was born."
Deco arrived in Portugal in 1997 and obtained Portuguese citizenship in 2002. Uncapped for Brazil, he was eligible to play for Portugal and in March of the following year Luiz Felipe Scolari selected him to play in a friendly against Brazil.
Deco scored an 82nd minute winner and has gone on to earn 71 caps and score five goals for his adopted country.
"Given my career, the most normal thing would have been to play for Brazil. There are players who didn't achieve half of what I have, but who wore that (Brazil) shirt," he said.
"Playing in the Brazil team would have been the easiest and most natural thing for someone who won everything with Porto, was in Barcelona's starting XI for four years and plays at one of the main clubs in the world (Chelsea)."
The World Cup finals will be the last time Deco plays for Portugal as he wants to spend more time with his family and possibly return to Brazil after his contract with Chelsea ends at the end of next season.
The 32-year-old said he wanted to reach the finals in top form.
"I believe, honestly, that I am going to have a great World Cup", he said.
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