The Blues are in pole position to land the striker.
They are due to meet Atletico Madrid representatives next week. But a top Atletico source said: "This is a battle between Chelsea and Inter.
"If the club decide to let Sergio go in January then he will go to one of those two clubs."
Aguero, 21, is top of boss Carlo Ancelotti's hit-list and Atletico have admitted they would be willing to off-load the Argentina superstar.
The Spanish outfit turned down the Blues' £43m bid back in August, as exclusively revealed by SunSport.
But they have admitted since that not selling Aguero was a mistake.
His get-out clause is closer to £50m but Atletico accept no one will match it.