كتب : Amr Fahmy
The victory boosted Milan’s UEFA Champions League hopes as they lie fifth with 33 points, two adrift of fourth-placed Empoli.
Ronaldo needed only 15 minutes to put the Rossoneri in front after he received Andrea Pirlo’s inch-perfect cross with a looping header past Siena's Austrian keeper Alex Manninger.
Four minutes later, Siena grabbed the equalizer through Simone Vergassola who made use of Cafu's poor positioning to hit a low drive home from a close range.
Ricardo Oliveira restored the lead for Milan in the 29th minute when he latched on to Ronaldo’s through pass before slotting into the back of the net.
Just two minutes later, Siena’s January-signing Massimo Maccarone leveled the score again from inside the box, thanks to Kahka Kaladze's poor tackling.
Milan midfielder Cristian Brocchi was sent-off in the 78th minute as a result of a reckless challenge on Ciccio Cozza.
Nine minutes from time, Ronaldo scored his second, tapping in Kaka's cross into the empty net from a close range.
Italian striker Maccarone looked to have secured a precious draw for the hosts with a superb shot from inside the area one minute before the final whistle.
However, Massimo Ambrosini sealed the hard-fought victory in stoppage time heading home a corner kick.