Two second-half strikes from the San Siro duo take Massimiliano Allegri's men to the top of the division, whilst the Bianconeri suffer their sixth straight defeat
A sweetly struck opener from Nocerino put the hosts in front on 54 minutes, and an Ibrahimovic penalty 10 minutes later confirmed the three points to temporarily send the Rossoneri to the top of Serie A.
Fresh off the back of their disappointing 2-2 draw with Bologna last Sunday, and clearly eager to make amends, it was Milan who started the brighter of the two sides; Boateng and Ibrahimovic, in particular, looked liveliest within the opening few minutes.
And although it was all Milan throughout the opening exchanges, the first real chance of the game fell to Siena. Reginaldo cut inside from the left wing, and spanked a shot past Marco Amelia, only to see his effort go agonisingly wide of the post.
It was soon becoming clear that Siena were happy for Allegri's side to dominate possession, and then catch the Rossoneri on the break. And that was very much the case on 23 minutes as Francesco Bolzoni found himself free inside the Milan box after a swift breakaway and flicked the ball past the on-rushing Amelia, but also inches past the post.
That was to be the best chance of the half, as both sides went into the break level with Giuseppe Sannino and his men nullifying Milan perfectly.
The second period was looking to play out in exactly the same fashion as the first: Milan dominating possession, and Siena replying with quick counter-attacks. But whereas in the first 45 Milan were unable to fashion a truly exceptional goalscoring opportunity, early in the second they did and duly took the lead. On 54 minutes Nocerino received the ball off Robinho on the edge of the area, and smashed this shot low into the bottom right-hand corner to leave Brkic rooted to the spot.
With Siena desperate to equalise, it was Milan's turn to hit them on the counter-attack. And with Boateng soon bearing down on goal, Brkic came out to stop a shot, but tripped the former Tottenham man and conceded a penalty. It was Ibrahimovic who took responsibility for the spot-kick, and smashed it low into the bottom left-hand corner of the net to double the hosts' lead.
Pato almost made it three with 15 minutes to go, but his shot was blocked on the line, and from there the game fizzled out as Allegri's side saw out a 2-0 victory.
Milan provisionally move to the top of the table but could drop back to third if Juventus and Udinese win their games on Sunday. The Rossoneri will end 2011 with a league clash away to Cagliari on Tuesday, while Siena host Fiorentina still in a relegation dog-fight.