World Cup 2014: Cristiano Ronaldo could miss Portugal's opening game after limping out of training with an ice-pack on his knee

Star is a doubt for their opener against Germany on Monday

Cristiano Ronaldo covers his face as he limps out of a Portugal training session (Getty Images)

Cristiano Ronaldo is a doubt for Portugal's World Cup opener after limping out of training overnight with an ice pack on his knee.

The world’s best player struggled with injury towards the end of the season and was unable to play in two of Portugal’s warm-ups with knee and thigh complaints.

The Real Madrid star returned in the 5-1 win against the Republic of Ireland earlier this week, but had to leave the squad’s training session after just 20 minutes.

He took part in physical exercises but not sessions that included ball work. The Portuguese football federation confirmed last week that Ronaldo had been suffering with tendinitis.

Ronaldo, 29, missed six of Real Madrid's final 12 fixtures and though he played in the Champions League final, scoring the fourth from the penalty spot, he was far below his best.

But team-mate Joao Moutinho played down concerns about the knee injury, saying: "Ever since he started to train with us he has been 100% like the rest of us.

"Everything is fine with him but there is some caution, not only with him. Many other players were using ice as well.

"There is nothing to worry about. The ice is normal after training."

Paulo Bento's Portugal face Germany on Monday.