The duo faces each other in the Copa America final on Sunday.
Former Barca superstar Neymar spent four years at Camp Nou playing alongside Lionel Messi where they formed a fearsome attacking trio with Luis Suarez.
But in 2017, the Brazilian sealed a world-record £198million transfer to Paris Saint-Germain.
On Sunday early morning, both players search for their first Copa America trophy.
“Messi is, as I’ve always said, the best player I’ve ever seen play and he’s a great friend, but now we’re in a final, we’re rivals,” said Neymar ahead of the final.
“I want to win and I really want to win this title, which would be my first Copa America.
“Messi has been looking for his first title with the national team for many years, and every time we [Brazil] are not present at the tournament, I cheered for him. That’s what I’ve been cheering for in the 2014 World Cup final, when he faced Germany.
“Now Brazil is in the dispute, so our friendship is on the line. The respect between us is still great but only one can win.
“When you’re friends with someone, it’s hard to forget the friendship you have – but for example when you play video games with a friend, you want to beat him anyway. It’ll be the same thing on Saturday.”