The Brazilian combined with Mario Balotelli nine minutes after the interval, sending a sumptuous curling effort into the top right corner in a true moment of quality in an otherwise dour affair.
It was Kaka's first goal of the season, and one incredibly similar to a strike he netted against the same opponents in a 4-1 win back in September 2008.
Kaka was given just his second league start of the season as Massimiliano Allegri made four changes from Sunday's 3-2 defeat at Parma, while Lazio boss Vladimir Petkovic also rotated heavily.
Just five Lazio players who started the club's 2-0 win over Cagliari survived, with German striker Miroslav Klose – who came off the bench to score on Sunday – among the new inclusions.
As the half wore on the home side assumed total control, but Milan's Riccardo Montolivo saw two thumping drives easily handled by Marchetti.
Klose wasted Lazio's first real chance of the game five minutes after the break, with the veteran's touch letting him down when better control would have seen him with just Milan goalkeeper Gabriel to beat from eight yards.
Four minutes later Kaka put Milan in front, receiving Balotelli's pass from the left before curling an unstoppable shot beyond a helpless Marchetti from 20 yards.
French defender Caini was the scorer, rising unchallenged to glance home Candreva's cross from the right from six yards.
Klose, with eight minutes to go, had a chance to secure Lazio's first away league win against Milan since August 1989, but his tame effort was straight at Gabriel, as both sides shared the spoils.
