Last updated on .From the section European Football

Guarin made his move from Porto to Inter Milan permanent in 2012 after an initial loan

Mario Balotelli returned to action for AC Milan but could not prevent Inter Milan winning a closely fought derby.

Balotelli, back at the club on loan from Liverpool, hit the post after coming on as a substitute as a goal from Fredy Guarin moved Inter top.

Roberto Mancini's side maintained their 100% start by continuing their impressive recent record against their city rivals at the San Siro.

Colombian midfielder Guarin found the bottom corner from outside the area.

Is Mario back on track?

Mario Balotelli managed one Premier League goal last season for Liverpool

Without question, Milan carried much more of a goal threat after Balotelli's introduction just after the hour, and the 25-year-old drew praise for his performance from team-mates and his manager afterwards.

"The Mario Balotelli we saw tonight is the one we want to see every match," said AC Milan captain Riccardo Montolivo.

"He is training well. Let's give him time and then come Christmas we will see how he is getting on and how much he has changed."

The former Inter striker was immediately into the action, taking responsibility for set-pieces, striking the frame of the goal from outside the box and forcing a fine save from Inter goalkeeper Samir Handanovic.

Equally as importantly as he attempts to atone for an abject 12 months at Anfield, the Italy international refused to respond to Inter's attempts to provoke him during an encouraging cameo.

By doing so, he seemed to show the mental strength he says he requires to get his career back on track again. It has been reported that he has a good conduct clause external-link in his contract, meaning he is banned from extravagant haircuts, smoking and visiting night clubs.

"He definitely did well after he came on," added Milan coach Sinisa Mihajlovic. "He is training well and will surely give us a big hand. He knows what he has to do, he has to continue working and now is not the time to judge him."

Are Inter serious title challengers?

Juventus have won the last four Serie A titles, but their shaky start to the season - one point from their opening three matches - could open the door to a title bid from another team this season.

And Inter could be that side, having made their best start to a season for 13 years, returning to the summit of Serie A after the latest victory.

It is now just one defeat in nine games against their fierce rivals, and Sunday provided further evidence of their ability to grind out wins when not playing to their maximum.