Internazionale's march towards the title continued with their 16th league victory in a row after edging past Cagliari 1-0 at the Stadio Giuseppe Meazza.
Nicholas Burdisso scored the only goal of a relatively uneventful game, which increased Inter's lead at the top of Serie A to 14 points.
Second place Roma also lost ground on the run-away leaders after losing out to fourth place Empoli 1-0 at the Stadio Carlo Castellani.
Empoli took the lead after only five minutes when Nicola Pozzi scored the only goal of the game to give the Tuscan club all three points.
Milan broke Siena's hearts with a late strike from Kaka that finished an eventful game 4-3 at the now re-opened Stadio Arternio Franchi.
The spectators, who missed out on the action last week, were treated to an entertaining match that saw seven goals hit the back of the net.
Milan took the lead after Ronaldo scored his first goal for his new club on only his second match after joining from Real Madrid.
Simone Vergassola then brought the home side level just three minutes later before Ricardo Oliveira restored Milan's lead on 29.
Former Middlesbrough striker Massimo Maccarone then hit back almost instantaneously with a goal scored straight from the kick-off.
The ever impressive Ronaldo looked to have secured all three points for Milan with a goal inside the last ten minutes.
But Siena hit back yet again through Maccarone only to see Kaka score with the last kick of the game. Click below for individual match summaries:
Empoli 1-0 Roma
Inter 1-0 Cagliari
Siena 3-4 Milan