Joachim Low's men face the fellow Euro 2012 semi-finalists in their opening game of Group G hoping to gain an advantage ahead of further clashes with Ghana and USA.
Reus, who is featuring in his first World Cup this summer, stressed the quality of the Portugal captain and all-time top goalscorer and believes every tie in Brazil will be tough.
"Portugal are an extremely strong team, with an outstanding individual player: Cristiano Ronaldo" the Borussia Dortmund attacker said in a new self-interview to mark the launch of PUMA Tricks. "He can decide a match on his own.
"USA coach Jurgen Klinsmann has lots of experience and he has built a solid team. As we saw at the World Cup in 2006 [when he coached Germany to the semis], he can take teams far in the tournament.
"There won't be an easy opponent."
Reus is in contention to be a key player in the starting XI for Germany this summer and insists he always had faith that he could feature in the tournament and play a role in winning the trophy.
"Yes, of course [I always believed I'd play in the World Cup], you have to believe in yourself," the German added. "It's vital not just in football but in day-to-day life.
"You also need a bit of luck, you need talent and determination. But you definitely need belief, that's for sure. It's my first World Cup so I'm feeling really motivated. I'm eager to play a big part and start as many games as possible."
Reus is among a number of global superstars who will be sporting the new PUMA Tricks boots in Brazil this summer.
For more information on the PUMA Tricks boot and Nature of Believing, visit PUMA FC.