Real Madrid star has struggled with injury since the Champions League final but has 'no worries' about his fitness for Brazil
The 29-year-old has been dogged by multiple injuries since Real Madrid's Champions League final win over Atletico, and there were concerns he could join Marco Reus, Franck Ribery and Radamel Falcao in missing the tournament.
However, the former Manchester United winger appeared from the start against Martin O'Neill's side in New Jersey on Tuesday night and completed 65 minutes before being replaced by Nani.
It has left Ronaldo confident he will be firing on all cylinders when Portugal face Germany in their opening World Cup game on Monday.
He told Times of India: "I am training well, keeping the advice of the doctors in mind. The progress is encouraging, they say, and I am feeling better with each day.
"You need to reach there when you are at your peak, both physically and mentally. I shall definitely be there, fully fit, no worries."
Paulo Bento's side face a tough task to qualify from Group G, after being drawn alongside Germany, USA and Ghana, and Ronaldo is realistic about the challenges ahead.
He added: "We played Germany in the Euros, so we know a bit or two about them. On the other side of the coin, they know more about us too. They have a great side. They are one of the favourites to win it this time. It's going to be really tough.
"But we had the Netherlands and Denmark in our group in the Euros too and we did qualify. This time we have the USA and Ghana. It's going to be real tough. But we are determined to give our best."