Portugal captain Cristiano Ronaldo trained on Friday ahead of the vital World Cup group match with USA as speculation swirled over his injured left knee.

The world player of the year, who is suffering from tendonitis, had the knee taped as he took part in light drills at the team's training camp near Sao Paulo.Ronaldo, 29, caused further concern over his fitness when he stopped training early and had ice applied to his knee at a public session on Wednesday.

On Friday, the Real Madrid forward showed no signs of discomfort as he took part in a warm-up passing exercise with team-mates Nani, Miguel Veloso, Helder Postiga and Ruben Amorim.Watching media were ushered away after 15 minutes but Ronaldo's appearance raises the likelihood he will play Sunday's game, which has great significance after Portugal's 4-0 loss to Germany.

Reports have been circulating questioning the fitness of Ronaldo, who is considered critical to Portugal's chances of success at the World Cup.After Real Madrid's win in last month's Champions League final, Ronaldo missed two Portugal friendlies before returning to action in their final warm-up against Ireland.