Chile forms part of Group B, along with Spain, Holland and Australia, who they play on Friday in their first match of the championship, reported Xinhua. (Agencies)
"We are preparing really well, we are training in very good form and very intensely, keeping in mind our rivals Australia, who are perhaps the most important rivals we are going to have," said Diaz, who plays for Swiss team Basilea.
Speaking to Chilean media at the team's training camp in Brazil, in Toca da Raposa, Monday Belo Horizonte, the former Universidad de Chile player said: "Australia are a great team that plays football very well, they are not an easy team" to beat.
Diaz's remarks came as a surprise, since group phase rivals Spain and Holland are widely considered stronger teams, with Spain winning the last 2010 World Cup in South Africa, and Holland taking second place.
Asked whether midfielder Arturo Vidal, the winger who plays for Italy's Juventus and recently had knee surgery, will be playing Friday's match, Diaz answered by praising his likely substitute Felipe Gutierrez, who plays for Holland's Twente.
"We have to wait and see if he will be able to arrive for these games, which would be very significant," Diaz said."No matter who plays Friday, he will do well," Diaz said, adding "while it's true that Felipe Gutierrez is one of the youngest players in the line-up, he has a lot of experience, he has been playing in Europe for a couple of seasons."
Gutierrez did not play much in the qualifiers, but when he did, he did well, and "that made (coach) Jorge Sampaoli and us very happy, because we have a player who's in great condition and who can come in at any moment", said Diaz.
Chile plays Australia on day two of the World Cup at Cuiaba's Verdao Stadium. Their second game will be on June 18 against Spain at Rio de Janeiro's Maracana Stadium and the final group phase game will be on June 23 against Holland at the Sao Paulo Arena.
Chile forms part of Group B, along with Spain, Holland and Australia, who they play on Friday in their first match of the championship, reported Xinhua.