Following Milan's 1-0 loss at Palermo on Saturday, Giampaolo Pazzini's 52nd-minute strike was enough for a sluggish Inter to claim the three points against Lecce at the San Siro.
Later, Edinson Cavani's double in a 2-1 win over Cagliari re-awakened Napoli's title challenge.
"The derby won't decide the title - there are 21 other points in play," Inter coach Leonardo, who replaced Rafael Benitez when the gap to season-long leaders Milan was 13 points, told Italian broadcaster Rai.
"It doesn't matter who is in better form going into the match because the derby is such a one-off. It's a difficult game to predict and especially strange to prepare for with the international matches."
Napoli, who appeared to have thrown away a shot at the title after losing to Milan 3-0 three weeks ago, closed to within three points of the leaders with victory in Sunday's late match.
But Cagliari, thrashed 4-0 by Udinese last week, equalised seven minutes later through Robert Acquafresca's clever finish. Five minutes on though, Cavani slotted home his 22nd goal of the season to claim the points after a fine run from Marek Hamsik.
In Sunday's early match, Roma and Fiorentina played out an entertaining 2-2 draw at the Artemio Franchi stadium where Totti's second double in a week pegged back the improving visitors who had twice gone ahead in the first half through Adrian Mutu and Alessandro Gamberini.
The draw maintains coach Vincenzo Montella's unbeaten start in the league as Roma stay sixth, six points off the Champions League spots. Totti, now on 201 goals in Serie A, is within four of Roberto Baggio who lies fifth in the all-time list.
