The Rossoneri made a blistering start and raced into the lead within 30 seconds of kick-off, as Davide Calabria broke clear to latch onto a through ball from fellow full-back Theo Hernandez and tap in, after seeing his first effort parried back to him.
Atalanta sought to hit back and Ruslan Malinovskyi forced a desperate block after Duvan Zapata’s shot was saved. On the stroke of half-time Milan doubled their lead though, as Sandro Tonali robbed Remo Freuler at the back and drove through in space to calmly fire in.
The Rossoneri looked to have put the result beyond all doubt late on, as a rapid counterattack culminated in Rafael Leao drilling into the top corner after being teed up by Hernandez. Atalanta were then awarded a penalty after a Junior Messias handball, allowing Zapata to blast in, before Mario Pasalic made Milan sweat by tapping in deep into stoppage time.