For the past decade, the battle to be crowned world's best footballer has been between two players: Cristiano Ronaldo and Lionel Messi.

Messi dominated the early stages of their rivalry, enjoying much success with Barcelona and winning the Ballon d'Or in four consecutive years from 2009-2012.

The tables have turned since then, though, with Ronaldo expected to win a fifth Ballon d'Or this year following Real Madrid's historic La Liga and Champions League double.

Should the inevitable happen, Ronaldo will equal Messi's record haul of five Ballon d'Ors and make it four in the last five years.

Their rivalry has entertained viewers for the best part of 10 years now and it's always been a wonder exactly what they think of each other.

Indeed, Ronaldo recently admitted he and Messi are not friends but that they respect each other, also describing the Argentina as a player he likes to watch.

"I'm not Messi's friend, but we're fellow professionals," he told Fox Sports Argentina. "We've shared many moments at award ceremonies in the last 10 years, and I have a very good relationship with him.

"We don't eat out together, but I respect him. He's a fellow professional of mine, not a rival. It's one more issue for the press, which want to sell papers and do their business.

"Watching Messi play? Of course I like it, I like to watch all the good players and he's one of them. I enjoy watching him play.

"How can I not like a person who hasn't done anything wrong? On the contrary. When Messi is asked about Cristiano, he also speaks well of me. My relationship with him is cordial and we respect each other."

Messi has now returned the favour by revealing what he really thinks of Ronaldo as a player and it would appear the feeling is mutual, describing the Portuguese as "phenomenal".

"No, I have always said it -- and [Ronaldo] from his part -- that he is armed with the presence of us both," Messi told Tencent, per ESPN.

"We try to achieve the best every year for our team, and what is said outside of that I don't think is very important.

"He is a phenomenal player with a lot of quality. All the world knows, and that is why he is one of the best of the world."

Ronaldo and Messi might not be friends, but it just goes to show their class that they still respect and praise each other.