Three-time World Cup winners Italy, favourites to cruise through group G, have been warned by coach Giovanni Trapattoni to guard against complacency and their rivals' threatening ambitions and form.
Croatia, who finished third on their finals debut four years ago in France, may be ageing, but remain a clever and dangerous team.
Revived Mexico have fresh momentum, and unsung Ecuador, making their maiden appearance, have confidence after out-qualifying Brazil.
"There will be surprises, as always," said Trapattoni, 62, a wily and successful veteran of the European club game experiencing his first major international tournament as Italy's coach.
"Nothing is certain. Everyone will be tough to beat and we have to respect each team."
Italy, even with early fitness doubts surrounding playmaker Francesco Totti and striker Filippo Inzaghi, are the class act with an experienced, mature and skilful squad.
The Italians also have a formidable record in the World Cup.
They were eliminated only on penalties in the last three tournaments - including losing the 1994 final to Brazil -- and have lost just one game, in open play, since the second round of the 1986 tournament in Mexico.
In the final of the Euro 2000 tournament, they lost to France, but only by a golden goal in extra time.
Their sole World Cup defeat, in 19 outings in the final stages of the tournament, was by Ireland in their opening fixture at USA '94, in a group game.
Knowing that, they will be on their guard against Ecuador in Sapporo, on Monday, as they seek to extend their run.
In a group of intriguing possibilities, the two Latin American nations are the outsiders.
Croatia could prove unpredictable. The team that took them to a gloriously unexpected third place in Paris has aged, coach Miroslav Blazevic has retired and another 62-year-old, Mirko Jozic, has taken over.
Jozic has wide experience, including coaching the "golden" generation of players who carried Yugoslavia to triumph at the 1987 under-20's world championships in Santiago, and working in Chile, the United States, Mexico, Saudi Arabia and Argentina. He took the Croatia job after being promoted from youth development.
Having revised Croatia's system and revived their spirit with a counter-attacking format that enabled them to win five of their final six qualifiers, Jozic has earned respect.
"I wish we had five players who were younger, faster and better than Robert Prosinecki, but we don't," he said in a dry understatement that reflected Croatia's position. "If we reach the second round, I'll be happy."
Prosinecki, 33, recently with Portsmouth in the English first division, was once a glittering star at Real Madrid and symbolizes Croatia's creative dilemma. In attack, they are dependent on the fading pace and power of Davor Suker, 34, tournament top scorer in France with six goals.
Ecuador and Mexico are less known. Under Colombian coach, Hernan Dario Gomez, Ecuador overcame Brazil and delivered a romantic qualifying tale.
When a maniac shot Gomez was shot in the leg in 2001, he retreated to Colombia, but he answered an impassioned call to return when a song was written in his honour. Striker Agustin Delgado - known as El Tin - was joint top scorer in the qualifiers with Argentina's Hernan Crespo and could spring a surprise.
Coach Javier Aguirre revived a flagging Mexico last year to ensure qualification. He has a reputation for strong views and emotional outbursts, but his team, built round the experience of recalled midfielder Alberto Garcia Aspe, 35, who has won 108 caps, is unexceptional.
"I believe in our ability," said Aguirre. "We are going to have a good World Cup." In 11 finals appearances, the Mexicans have only reached the second round four times, including two appearances as hosts. Another success would be a surprise.