The victory books Lazio a ticket to next season's Europa League, much-needed for Simone Inzaghi's side
AC Milan are fifth, one point behind Inter, with Roma a further three points adrift in sixth before they host champions Juventus
The Polish star now has scored six times in four starts for Milan since moving from Genoa in the January transfer window to replace Gonzalo Higuain.
Carlo Ancelotti's Napoli missed the chance to cut the gap to the reigning champions after Inter Milan substitute Lautaro Martinez hit a stoppage-time winner in an ugly 1-0 victory.
Inter holds the third spot in the Serie A standings with 25 points, three points behind second-placed Napoli, while Atalanta provisionally climbed up to sixth place in the table with 18 points.
Jordan Veretout stepped up to slot past Pierluigi Gollini after 63 minutes, and Fiorentina cemented the win when Cristiano Biraghi doubled their lead in stoppage time.
Everton, who spent heavily in the close-season, were eliminated from the Europa League after Thursday's defeat to Atlanta.