The winner will qualify for the World Cup quarter-finals alongside already-qualified England.
The loser will miss out.
"We are well, knowing the importance that this match has for us," said Creevy.
"It is a round-of-16 decider. Another final that we have to overcome.
"We are confident and convinced of what we have in front of us on Sunday."
Creevy, in his fourth World Cup at the age of 38, added: "We are training very well, trusting in what we have been doing for a long time. We have to continue highlighting things and improving others.
"Personally, I am taking it very calmly, enjoying it game by game, and this week enjoying day by day. I think that if we do things well, the team will respond.
"If we want to be in the final, semi-final or quarter-final, we have to get through this match first so it's really important for us.
"We have to stay connected and believe in ourselves. Obviously, it is an important game, but not the most important in our history."
Losing, however, was not something in Creevy's mindset.
"You don't think about that," said Creevy, who skippered the Pumas to the 2015 World Cup semi-finals.
"We are preparing to win the game. Then there are consequences, unfortunately.
"We prepare to win the games, we see the important points where we can improve, where we can continue working, believing in ourselves, in what we are doing.
"You always think about how you win and not what might happen if you lose."
Argentina lost 27-10 to England in their opening match before rebounding with wins over Samoa (19-10) and Chile (59-5).
Japan overcame Chile 42-12 before losing 34-12 to England and then beating Samoa 28-22.
Defence coach David Kidwell said the match against the Japanese was "another important game for us".
"Japan are very well coached, they have good skills. In our defence, connection and making tackles are important," he said.
"Japan put a lot of people on their defensive line and it will be a tough game. We have to focus on what we do well."
Full-back Juan Cruz Mallia said Japan "are a very neat team, very methodical, they move their legs a lot, they have very good defensive organisation".
"The back three are really fast, their fly-half kicks very well and the forwards are also good. They are going to be really tough on Sunday".
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