Now they're both into their 30s, it's inevitable that Lionel Messi and Cristiano Ronaldo's legendary rivalry will start to slow down.

For well over a decade now they've been fighting tooth and nail for the title of world's best player, with both players winning the Ballon d'Or on five occasions.

Who is really the best is a purely a matter of opinion. Messi is arguably a more complete player, but then Ronaldo is the perfect marksman.

It could ultimately come down to this year's Ballon d'Or. With the likes of Mohamed Salah and Neymar creeping up behind them, 2018 might be Messi and Ronaldo's last chance.

As it stands Ronaldo is bookmakers' favourite because of Real Madrid's run in the Champions League, while Salah is second following a stunning debut season at Liverpool.

Messi, perhaps surprisingly, is third favourite despite being Europe's leading scorer on 34 goals and guiding Barcelona to the La Liga and Copa del Rey double.

What's so great about Messi and Ronaldo's rivalry is that there's no animosity. On the pitch they're fierce competitors, but there's an underlying respect between them.

THE ULTIMATE RIVALRY

Indeed, both the Argentine and Portuguese superstars have previously waxed lyrical about each others' ability and achievements.

But the fact is they're rivals, so it comes as no surprise that Messi's name is actually banned in the Ronaldo household.

In an interview with L'Equipe, Ronaldo's sister, Katia, revealed that the 'M-word' is never spoken because Ronaldo likes to live in his own bubble and feel protected.

"We don't talk about Messi inside the house," she said. "Cristiano knows that outside the door, there's a whirlwind.

"And he knows that when he crosses the door from the outside to the inside, he is protected. This is where he recharges all his batteries.

"He's very focused, in his own bubble, but he's also very anxious. He plays the match out in his head, thinks about how things will happen.

"There are some who play until the age of 37. Cristiano is not a machine, but he will continue to play while he can, I think for three or four more years.

"Cristiano has always been a perfectionist, he's always seeking to be the best, to do everything to win. But aside from that, he is really reserved."

Messi's name being banned is understandable, but one imagines that must be tough for Ronaldo's son, Cristiano Ronaldo Jr, who has previously described the Barcelona star as his "idol".