Spain will be the favorite to add another World Cup to its trophy case - this time the Under-20 version - when the tournament begins on Friday.

Ghana, which upset Brazil two years ago in the final in Egypt, has failed to qualify this time around. But six-time champion Argentina is back after missing out in 2009 and will be among the contenders with four-time winner Brazil, France and a long list of challengers in a strong 24-team field.

Some players are already with big European clubs. Juan Manuel Iturbe just moved to Porto in Portugal, while teammate Erik Lamela is joining Roma from Argentine side River Plate. But new talent is bound to be uncovered in the 3 1/2-week tournament.

The biggest concern for many teams may be dealing with the ''thin air factor,'' with six of the eight venues at altitude.

Cartagena and Barranquilla are at sea level on the Caribbean. The other six venues are at altitude: Bogota (2,600 meters/8,600 feet), Manizales (2,150 meters/7,100 feet), Medellin (1,500 meters/5,000 feet), Armenia (1,500 meters/5,000 feet), Pereira (1,400 meters/4,600 feet) and Cali (1,000 meters/3,300 feet).

''We trained at altitude before arriving in Colombia,'' France coach Francis Smerecki said. ''In Bogota, we have been finding out some things and the players are getting used to it.''

Studies have suggested that a free kick from 20 meters (yards) will reach the goal about 5 percent faster than at sea level. Most players probably need several weeks to adjust, although some countries like Mexico may need less time since many cities in the country are at similar altitudes.

Spain will be led by three players already with giant European clubs: Dani Pacheco (Liverpool), Sergio Canales (Real Madrid) and Oriol Romeu, who will move from Barcelona to Chelsea pending agreement of personal terms.

''Playing in the tournament is great, and winning it would be incredible,'' said Canales, who is expected to miss the opening match against Costa Rica because of a leg injury

Spain coach Julen Lopetegui closed some of his practices leading up the opener and prohibited taking photos.

''There are no easy groups in the tournament,'' Lopetegui said, ''and every game will be an adventure.''

Brazil will be without two of its stars, striker Neymar and midfielder Lucas, who led the team to the South American championship earlier in the year. They were released from the competition because they played in the Copa America for the senior team. Brazil coach Ney Franco also will be without striker Roberto Firmino, who was not released by German club Hoffenheim.

But the majority of the team has been together for a bit.

''We are at an advanced stage because these players have already played together,'' Franco said.

Brazil's young stars include Philippe Coutinho and Casemiro.

''I hope to play my game and end up as champions,'' Inter Milan midfielder Coutinho said. ''My game is to help out in midfield and find space to pass to the forwards.''

Argentina has won five of the past eight Under-20 titles, guided by stars like Lionel Messi and Sergio Aguero. Iturbe is under some pressure by being hailed by some as the next Messi. Coach Walter Perazzo will also have Lamela, Facundo Ferreyra, Roberto Pereyra and Adrian Cirigliano.

Perazzo's job could also be on the line if Argentina does not reach the final four. This would be another blow to the country's football image following a poor performance by the senior team in the Copa America, which led to the firing of coach Sergio Batista.

''The phrase pressure is used a lot,'' Perazzo said Thursday. ''We have to play this World Cup and enjoy the fiesta of football. We know that in top competitions, these kind of shake-ups are likey to take place. One never knows when, but one has to know how to handle it and separate yourself from the negative stuff.''

England will be missing some of its top players due to commitments with their club teams, including Arsenal midfielder Jack Wilshere. The tournament is outside the FIFA calendar and conflicts with the start of the English league season.

''We don't have mandatory release of the players, which means there are a number of players who won't be released by their clubs,'' England coach Brian Eastick said. ''So we accept that. Even if we did have mandatory release, we would still have to be sympathetic to some of the clubs and their players who are playing first-team football at the top level.''

England is in one of the toughest groups, with Mexico, Argentina and North Korea its opponents.

Others players to watch include Gael Kakuta of France, Portugal's Nelson Oliveira and Mexico's Erik Torres.

The tournament opens Friday with four matches, with the same number on Saturday and Sunday to complete the first round.

The curtain-raising match is Friday in Group F, with England against North Korea in Medellin, followed by another Group F match between Argentina and Mexico. In Group E on Friday, Brazil faces Egypt in Barranquilla and Austria plays Panama in Cartagena.

On Saturday, it's: Mali vs. South Korea and Colombia vs. France in Group A; plus Cameroon vs. New Zealand and Portugal vs. Uruguay in Group B.

On Sunday, it's: Costa Rica vs. Spain and Australia vs. Ecuador in Group C; plus Nigeria vs. Guatemala and Croatia vs. Saudi Arabia in Group D.

The group winners and runners-up advance to the round of 16, along with the best four third-place teams.

The round of 16 is Aug. 9-10, the quarterfinals Aug. 13-14, the semifinals Aug. 17 and the final and third-place game on Aug. 20 in Bogota.