The Scudetto holders will be disappointed not to mark the opening defense of their title with a victory, as an emphatic opening 45 minutes promises to set the tone for the season.
The visitors took a shock lead with goals from new arrivals Miroslav Klose and Djibril Cisse, but Zlatan Ibrahimovic and Antonio Cassano ensured the Rossoneri did not begin the new campaign with a whimper.
Lazio began to pile the pressure on its northern Italian counterparts just minutes into the game, and it proved fruitful as ex-Bayern Munich star Klose opened the scoring for his side, by capping his debut with a sublime finish in the 12th minute.
Stefano Mauri floated an curling ball into Klose's path, and the German hitman collected the ball magnificently, showing great footwork to weave his way around Alessandro Nesta and Thiago Silva in the box and coolly drive his shot past Christian Abbiati.
Indeed, it got worse for the Scudetto holders only six minutes later, as Cisse made it 2-0 by meeting another of Mauri's impressively pinpoint crosses from the right, and powering a bullet header past the once-more-helpless Abbiati in the Milan net.
It would have been foolhardy to write off the current champions so soon, despite their defensive failings early on.
This was vindicated in the 29th minute when Ibrahimovic opened the new season with a simple tap-in, after fantastic build-up play from Aquilani, whose visionary first-time through ball fed Cassano through. He expertly drilled the ball hard and low into the Swedes path.
Cassano continued to display his credentials as a forward, rather than a trequartista, of the highest order, and he showed his predatory instinct in front of goal to grab an equalizer for the Rossoneri on 33 minutes. Aquilani again featured in the buildup, whipping in an inswinging corner, as it proceeded to meet the head of the No. 99 and the back of the net.
The second half offered far less invention and entertainment in comparison to the frantic first 45, as both teams took their foot off the pedal, content to maintain a slower tempo for the remainder of the match and much to the disappointment of the fans at San Siro.
Cassano's explosive right-footed shot rebounding off the post was the only real chance to develop in the midst of the half, but Milan then failed to profit from a late surge at the Lazio goal, after Ibrahimovic and substitute Alexandre Pato fluffed great chances to beat Bizzarri from close range, as both sides share the spoils in the first round of this season's Serie A fixtures.