The duo are on the same side this week as Brazil take on Russia and Germany in international friendlies.
But in next month’s Champions League quarter-final between Real Madrid and Juventus, both players will firmly be focused on securing qualification for their side.
And Casemiro admits he will show no mercy against his fellow countryman.
“It’s Madrid vs Juventus, in the 85th minute. Costa runs down the wing and you’re the only defender left: Do you think about the World Cup?” he was asked at a press conference this week.
“I will break him in half!” brutally responded the midfielder.
Costa is likely to be a regular starter for Brazil in the coming months due to Neymar’s recent injury.
The winger fractured a metatarsal against Marseille last month and is facing a race against time to prove his fitness for this summer’s tournament.
Speaking recently, Casemiro sparked rumours that Neymar may be moving to the Spanish capital this summer.
“He knows that the door is always open for him in Madrid, but he is happy in Paris,” he said.
“He always says that he is happy there, that he is comfortable in Paris.
“There are many Brazilians there too, it is very quiet there, but whether it’s possible you have to ask Florentino.”