Milan took the lead in the final minute of the first half when Rafael Leão burst down the left and exchanged passes with Théo Hernandez before curling into the far bottom corner.
However, Koopmeiners levelled almost immediately and then fired Atalanta into the lead from the spot in the 59th minute after Aleksej Miranchuck had been fouled by Milan teenager Alejandro Jiménez in the penalty area.
AAP