Real Madrid star Cristiano Ronaldo is confident he can keep going "for years to come" at the elite level of football.

The Portugal captain, who turned 33 on Monday, has shown signs of a decline in his form this season for the first time in his glittering career.

The forward has only scored eight goals in 17 LaLiga matches in 2017-18, four of which have come in Madrid's last two matches, although he has nine in six games in the Champions League.

But Ronaldo has dismissed any concerns about his ability to maintain his extraordinary standards in the future.

"On a personal level, this is a fantastic time for me," he told GQ magazine. "I have a growing family and I am very happy with my life.

"On a professional level, I have had two very good years in which I have won titles with my club and with my national team. I feel that I can keep going at the highest level for years to come."

Ronaldo does admit he has had to make changes in his lifestyle in order to maintain his fitness in recent years.

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"Life is a challenge from all perspectives and I try to be at my peak, physically, because this is very important in my profession," he said. "I have to be at 100 per cent and I take this very seriously

"To maintain this level, you need to make sacrifices. There are certain things that I could do when I was 20 that I can't do today. You have to try and find a balance. It's the finer details that make a difference.

"I am ambitious, very ambitious. People should chase their dreams. I chased mine and I want to keep doing so, to improve and achieve even more goals. I believe I've had some memorable performances throughout my career but can't recall any in particular. I try to focus myself more on the present than on the past."