Both sides struggled to create chances in a tight first half, with Roma just edging proceedings as Gervinho failed to turn a free header back across goal from close range.
The derby sprang into life after the break when Michael Ciani clipped the crossbar for Lazio, before Balzaretti smashed the upright at the other end.
Just moments later Balzaretti made his second effort count as Roma broke the deadlock, with the full-back watching Francesco Totti's chipped cross all the way before hammering home at the far post.
With 10 minutes to go, Lazio's task was made all the more difficult by a red card for Andre Dias, who was dismissed just moments after coming on when he hauled over Totti just outside the box.
Adem Ljajic had a free kick turned away by Marchetti as the Giallorossi dominated the closing stages, with Rudi Garica's side closing things out to maintain their 100% record.
In stoppage time Ljajic won a penalty kick after being pushed to the ground by Cristian Ledesma, with Ljajic stepping up himself to add gloss to the scoreline.