The Red Devils beat England 1-0 in Kaliningrad earlier this week to top Group H and will therefore face the Asian side in Rostov-on-Don on Monday evening.
Martinez knows all about Japan's strengths after grinding out a 1-0 friendly win in November and, despite Akira Nishino replacing Vahid Halilhodzic between then and the start of the tournament, the Spaniard is not expecting any surprises.
"We played a very entertaining game in Bruges not long ago," he told reporters. "There's been a change of coach but not a real change in the squad.
"They're very well organised, dynamic... technically, Japanese football is very strong and we expect a really competitive team. There won't be any big surprises.
"To be successful in a World Cup is not about facing the opponent that everyone expects is going to be giving you the golden key to go through.
"I don't think that in the World Cup you can be successful by trying to hope to get an easy path. We saw that in the Euros two years ago."
Belgium, beaten just once since Martinez first took charge nearly two years ago, will face either Brazil or Mexico if they reach the quarter-finals.