A fabulous late Miroslav Klose volley secured Lazio a fourth consecutive victory over Inter Milan.
Chances had been rare until the German striker made an intelligent run inside the box and was perfectly placed to connect with Antonio Candreva's cross.
Inter had enjoyed the best chance of the match up until then with Yuto Nagatomo wasting a fine opportunity.
The defeat ensures Inter remain sixth, eight points adrift of Napoli in third, while Lazio jump to 10th. Losing against a side who had previously won just twice in the last 12 games will be a setback to an Inter side hoping to close in on Napoli, who occupy the final Champions League qualifying spots.
The hosts defended superbly, reducing Serie A's second highest scorers to few clear chances, and deservedly edged the encounter for their sixth victory of the season.
Yuto Nagatomo will rue heading Jonathan's whipping cross straight at Etrit Berisha when an effort either side of the goalkeeper would have given Inter the opener.