Legendary French midfielder Zinedine Zidane - who now coaches B-side Real Madrid Castilla - believes Cristiano Ronaldo's incredible work ethic is what makes him one of the best players in footballing history.
Part of the Real Madrid forward's drive comes from his rivalry with Barcelona's diminutive Argentinian Lionel Messi, according to the 43-year-old.
Both players have dominated the World Player of the Year award - or its successor, the FIFA Ballon d'Or - over the past seven years, and often battle it out in setting new scoring records and firing their respective clubs to glory.
Despite working in close quarters with Ronaldo, Zidane is still shocked by the Portuguese star's relentlessness in front of goal.
"50 to 55 goals per season is just not human," Zidane told French TV programme Canal Football Club via ESPN FC.
"But you should see him at training. At training, he's a guy who every ball, every match, every corner ... he wants to score, he wants to win.
"And when he scores he does the same thing he does in a match. He's happy and he falls to his knees (to celebrate). it's incredible."
While both Messi and Ronaldo want to help their clubs compete for major honours, they inevitably match themselves up against each other as they compete to be the world's best player - and perhaps best player of all time.
"There's a feature in all of that and it's that there's another player opposite who pushes him. That's to say the rivalry he has with Messi.
"He doesn't need that (to perform), but what keeps him scoring 55 to 60 goals a season is the fact that he says to himself 'this year I have to score one more than the other guy.'"
Since his record-breaking move from Manchester United in 2009, the former Sporting Lisbon youth teamer has certainly established himself as a firm favourite in the Spanish capital.
"Since Raul left he's really become the other legend of the club," Zidane commented.
