Cristiano Ronaldo was warned by his doctor that he risks threatening his career by playing on for Portugal at the World Cup.

The Real Madrid superstar is struggling with a knee injury but is under pressure to lead his team into Sunday’s crucial clash with the USA after the 4-0 opening defeat to Germany.

According to Spanish online newspaper El Confidencial, Portuguese surgeon Jose Carlos Noronha told the 29-year-old: ‘Let your knee rest, or your future is in danger’, to which Ronaldo reportedly replied: 'I’m the one that says whether I’m playing or not.'

Ronaldo was pictured limping out of training with his knee heavily strapped.

There had been serious doubts over the Real Madrid superstar's fitness ahead of the tournament, after a thigh problem and tendonitis in his knee forced him to miss training and two warm-up matches.

Ronaldo declared himself '100 per cent' fit on the eve of Portugal's opening 4-0 demolition at the hands of Germany but, although he managed to play the entire game, he was largely peripheral and failed to change the course of the result.

It now appears that the Ballon d'Or winner has suffered a recurrence of his knee problems after receiving treatment during a training session in Campinas.

Ronaldo was taken through a series of stretches before having his left knee bandaged with an ice pack and helped off the field 30minutes before training finished. Ronaldo watched the rest of the session from the sidelines.