But Diego Forlan says his former team-mate is altogether different off it.
The pair spent four years together at La Liga giants Atletico Madrid.
And, speaking to the National, Forlan has now revealed what his fellow hitman is really like behind the scenes.
“It is like he transforms when he crosses the line.”
Diego Forlan opens up on former Atletico Madrid team-mate Diego Costa
The former Manchester United ace said: “One player who has been doing exceptionally well is Costa, the Premier League’s top scorer.
“We both joined Atletico Madrid at the same time in 2007 and I played alongside him a couple of times towards the end of my time at Atletico, but he was still a teenager when he arrived and kept being sent out on loan.
“He went to Braga, Celta, Albacate, Valladolid and Rayo Vallecano, which is normal for an emerging player who is not quite ready to play every game for a top team. My departure in 2011 helped create a space for him.
“I liked Diego. He was a funny guy, always joking about with the other Brazilian and Portuguese players there, always smiling and completely different to the man people see on the pitch, the fighter with the strong personality. It is like he transforms when he crosses the line.”
Costa has bagged 13 goals in 17 top flight fixtures so far this term.
And he’ll be hoping to find the net once again when Chelsea play host to Stoke this weekend.
Antonio Conte’s Blues currently sit top of the table having collected 46 points from their opening 18 fixtures.