Cristiano Ronaldo has been able to battle with Lionel Messi for the title of the world's best player because of his "strength and courage," his former Real Madrid teammate Iker Casillas has said.
Ronaldo, 32, helped Madrid to a La Liga and Champions League double last year and is favourite to win a fifth Ballon d'Or when France Football unveils the 2017 winner on Thursday.
That would see him equal the haul of Barcelona's Messi, 30, who has joined his Madrid rival in the top two of the poll for the last six years.
Casillas played alongside Ronaldo until leaving for Porto in 2015 and suggested that while the Portugal captain is "very different" to Messi, his work ethic has kept the two neck-and-neck at the top of the game.
"Regardless of whether you love Cristiano Ronaldo or Lionel Messi, there's no doubt that competing with a player such as Lionel Messi isn't easy," L'Equipe quoted him as saying.
"The consistency and the strength of Cristiano Ronaldo have enabled him to be one of the most important and recognised players.
"He's very different to Messi but, with strength and courage, he's also a world-class player."
Casillas played against Messi on numerous occasions during his Madrid career and said: "The thing, when you face Lionel Messi, is to stay organised. He's a very fast player, a livewire.
"In addition to scoring a lot of goals, he also provides a lot of assists. With Messi, you have to stay focused for the whole game, from the first to the 90th minute."