The 35-year-old striker met Gabriel Silva's floated cross with a perfectly timed mid-air volley that flew past befuddled Lazio keeper Federico Marchetti in the 19th minute.
The Udinese captain's 18th league goal of the season was enough to lift his side above Inter Milan and Lazio into sixth place, level on 51 points with fifth-placed AS Roma and seventh-placed Lazio.
In the day's only other Serie A game, Genoa missed the chance to move out of the relegation zone with a 1-1 draw at home to Atalanta.
Antonio Floro Flores' first league goal of the season in his first start after three months out injured gave Genoa a sixth-minute lead, only for it to be cancelled out two minutes later by a brilliant Cristiano Del Grosso strike.
Genoa move up a place to 18th on 29 points, one point behind Siena, who host Chievo Verona on Sunday (1300 GMT), while Atalanta are on 38 points, nine clear of the drop zone with five games left to play.
- Sports & Recreation
- Federico Marchetti
- Antonio Floro Flores