كتب : Sherif Tarek
The victory saw the Rossoneri close the gap on leaders and arch-rivals Inter Milan, who are now six points clear on top.
Siena's Gianluca Curci received a straight red card only after ten minutes to leave the minnows outnumbered for more than 80 minutes.
Two minutes later, Brazilian wizard Ronaldinho opened the scoring upon converting a penalty kick.
Marco Borriello doubled Milan's lead two minutes from the half-hour mark when he magnificently volleyed home an Andrea Pirlo cross.
Eighteen minutes from time, Ronaldinho headed a David Beckham cross into the net to revive his glorious days.
The former Barcelona superstar completed a hat-trick one minute before the final whistle.