Cristiano Ronaldo trains with knee brace ahead of Portugal's clash with USA

Cristiano Ronaldo again wore a brace on his troublesome left knee Friday, triggering a new wave of questions over his health as his Portugal team held it final World Cup training session before departing for a crucial game against the United States.

The reigning world player of the year took part in some light workouts with the first team, followed by stretching. He was smiling and appeared at ease at practice at the team base in Campinas, northwest of Sao Paulo, as he took part in a playful game of keep away from teammates. The team was set to leave later in the day for Manaus for its matchup with the U.S. on Sunday.

Portugal was humiliated in a 4-0 defeat by Germany in their opener. Another loss and the fourth-ranked team in the world could be eliminated even before its final Group G game against Ghana.

Ronaldo has been hampered by tendinitis and a muscle injury since before the tournament, creating plenty of speculation about how he would perform in Brazil.

He was doubtful to play before the Germany game, but declared himself fit and went on to produce a subdued performance that only added to the usual drama and scrutiny surrounding the player.

Team officials dismissed reports suggesting that Ronaldo could be risking his health by playing, saying they were tired of having to deny every rumor about a player many consider to be the world's best. They noted, however, that he often plays with a brace and said he's healthy and would play against the U.S.

Teammate Helder Postiga also expressed frustration over the media circus around Ronaldo.

"I have already answered so many questions about Cristiano Ronaldo. Actually we speak about him every day. If I am sick of it, imagine how he is feeling," Postiga said. "If he is available enough to train with us that's probably because he is feeling well. He's been training with us all these days and as usual he always gives his best. He's just fine."

Ronaldo has been one of the best players in the world for about a decade, and last year he even passed Lionel Messi by ending the Argentina forward's streak of winning the FIFA Ballon d'Or.

He has also excelled on the world stage, helping Portugal reach the 2004 European Championship final and the 2006 World Cup semifinals. The team also advanced out of the group stage at both Euro 2008 and the last World Cup, and then reached the semifinals at Euro 2012.

Although he is Portugal's all-time record scorer with 49 goals in 111 appearances, Ronaldo scored just twice in the last two World Cups.