Brazil striker Ronaldo has been training separately from the rest of the squad in order to build up his fitness.
The Real Madrid forward has not played since hobbling out of the Galacticos' 1-1 draw with Real Sociedad at the start of April and indicated that he needed more time to get to the same level as his international team-mates.
'As I've played less than them this season, I have to train more than them to be in form,' Ronaldo said.
'They have played the whole season and have had only one week break. I've had more free time but due to injuries, it's natural that I train more.'
At the training camp in Zurich, the whole squad trained in front of some 5,000 fans in the first of five training sessions that can be watched by fans in the camp, each of which is already sold out.
Meanwhile, Ronaldo's club-mate Roberto Carlos has spoken of his confidence about the upcoming World Cup: 'The environment is very good - we've been together for a long time and that helps a lot.'