Milan had the better of the opportunities in the first period, but could not make the breakthrough with the scores level at the interval.
It was a similar story in the second half, but Diego Costa secured a vital away goal for Atletico late on.
Read how it all unfolded in Sports Mole's minute-by-minute live commentary of the action below.
MILAN: Abbiati; De Sciglio, Rami, Bonera, Emanuelson; De Jong, Essien; Kaka, Poli, Taarabt; Balotelli
ATLETICO: Courtois; Juanfran, Miranda, Godin, Insua; Gabi, Suarez, Koke; Garcia, Turan; Costa
San Siro right now. pic.twitter.com/ynzT0i09fl— Milan Fans ★ (@MilanEye) February 19, 2014