Victory in the Derby della Capitale took the Biancocelesti clear of the Milan clubs and arch-rivals Roma; meanwhile, Monza finished March unbeaten to consolidate gains from earlier in their first Serie A season.
Match preview
Just days after becoming the first Italian club to be eliminated from Europe all season, any misery Lazio felt from their Europa Conference League exit to AZ Alkmaar soon evaporated with success in a close-fought Rome derby.
When Roma went down to 10 men following the first-half dismissal of Roger Ibanez - an event celebrated wildly by Sergej Milinkovic-Savic - Lazio sensed an opportunity to move further ahead of their old foes in the Serie A standings, and Mattia Zaccagni duly obliged with the winning goal midway through the second half.
Pulling them five points above their Giallorossi rivals, victory also left Maurizio Sarri's men as Napoli's closest contenders - albeit fully 19 points adrift of Italy's champions-elect - and they are well placed to secure a Champions League place for next season.
One of only two teams to beat Napoli in Serie A this term, Lazio have suffered just one loss in their last 11 league games and have compiled 16 clean sheets from 27 top-flight matches.
A total of 18 in the 2006-07 campaign represents the capital club's record in that regard - a tally which could soon be surpassed by Sarri's current side - and such a steadfast defence sees them ranked third across Europe's top five leagues for goals conceded.
One of those shut-outs came in November's 1-0 win over Monza at Stadio Olimpico, and the Biancocelesti will aim to complete the double over their Lombardy counterparts on Sunday.
Though more recent results have undercut their challenge for a top-half finish, Monza's Serie A debut has surpassed many expectations so far and they have already claimed several notable scalps.
Wins over Juventus both home and away were, of course, major highlights for an ambitious club who only made it out of Serie B via the playoffs and then started this season slowly.
The appointment of rookie coach Raffaele Palladino has transformed the Biancorossi's fortunes, and his team have lost just one of their last eight home games in the league - to regional rivals Milan.
After beating Empoli at the start of March, Monza were frustrated by two relegation-threatened opponents, and successive 1-1 draws against Hellas Verona and Cremonese dropped them down to 13th place.
Nonetheless, they are virtually safe already, as 15 points separate them from the relegation zone with 11 games remaining of a groundbreaking season in Brianza.
Monza Serie A form:
W
L
L
W
D
D
Lazio Serie A form:
L
W
W
W
D
W
Lazio form (all competitions):
W
W
L
D
L
W
Team News
After being sent off during post-match scuffles in the Derby della Capitale, Adam Marusic must sit out Sunday's game due to a ban, but Lazio captain Ciro Immobile is back following injury and should start.
Both Immobile and Mattia Zaccagni were omitted from the recent Italy squad for different reasons, but they are the joint-top Italian scorers in Serie A this season (both on nine goals). Set to start on the left of a front three once again, Zaccagni is aiming to reach double figures for the first time, while Immobile could do so for the eighth time in his last nine campaigns.
In addition to the latter's return, Manuel Lazzari will be offered an increasingly rare start deputising for Marusic at right-back, and Matias Vecino is in contention for a midfield spot after missing the Rome Derby.
Monza, meanwhile, will be without their captain, as Matteo Pessina joins Armando Izzo on the sidelines through suspension.
Marlon should slot into the hosts' back three in place of the latter, while Samuele Birindelli may return to the right flank, allowing Jose Machin to start in the engine room; Nicolo Rovella is not quite ready to return from an ankle problem.
Leading the line up front, Andrea Petagna has registered four Serie A strikes against Lazio in the past, including his first-ever goal in the top flight, back in August 2016.
Monza possible starting lineup:
Di Gregorio; Marlon, Mari, Caldirola; Birindelli, Machin, Sensi, Augusto; Ciurria; Caprari, Petagna
Lazio possible starting lineup:
Provedel; Lazzari, Casale, Romagnoli, Hysaj; Milinkovic-Savic, Cataldi, Alberto; Anderson, Immobile, Zaccagni