While Roma won 2-0 away at Chievo, Inter secured their victory thanks to Diego Milito's second-half goal.
Inter got off to the perfect start when Diego Milito cut inside and fired in with only three minutes gone.
Zanetti's Argentine compatriot Diego Milito finished coolly with the outside of his boot on 57 minutes after great work from his captain.
He fielded a three-strong attack of Mario Balotelli, Goran Pandev and Diego Milito, but they drew blank against a determined visiting side.
CNN: Inter held by Genoa in San Siro but stay four points clear
In other action, Inter Milan maintained new coach Andrea Stramaccioni's unbeaten start with a 2-1 win over Siena with Diego Milito scoring twice.
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Striker Diego Milito gave the home side the perfect start, taking advantage of some poor defending from John Terry and firing in after only three minutes.
BBC: Inter's Jose Mourinho confident of knocking out Chelsea
But Jose Mourinho's men may yet rue not having added to Diego Milito's well-placed strike from the edge of the CSKA area at the San Siro.
But Diego Forlan pulled one back after 71 minutes -- incredibly Inter's first in 541 minutes -- before the Uruguayan set up Diego Milito for the equalizer.
Sneijder exploited gaps on the Chelsea defense and from his pass the best chance fell to Argentine Diego Milito but he scuffed his shot wide of Turnbull and the goal.
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Four-time defending champions Inter took a 15th minute lead through top scorer Diego Milito, as the Argentine star eluded the attentions of several defenders to net his 10th of the season.
According to Spanish newspaper Sport, City's bid for Maicon could be rivalled by Real Madrid, whose new boss Jose Mourinho is also chasing Manchester United's Wayne Rooney and Inter Milan's Diego Milito.
The 26-year-old, who resurrected his career last season after moving from Barcelona, has finalized a deal that was agreed earlier in the summer and also saw Argentine striker Diego Milito move to the San Siro on Tuesday.
What followed was one of the most memorable Milan derbies in recent years, as Inter ran out 4-2 winners with Diego Milito scoring a hat trick, the first person do so in this fixture in almost six decades.
Samuel Eto'o was proving to be Inter's best outlet down the flanks but the Italian side struggled to break down the resolute German defence and with the Cameroon striker turning provider, they lacked a presence in the middle where Diego Milito was anonymous.
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