Red cards for Mauricio and Federico Marchetti gave the visitors a massive advantage which they were able to capitalise on despite missing a penalty
Hernanes came back to haunt nine-man Lazio as his double spared Inter's blushes in an action-packed 2-1 Serie A victory on Sunday.
The Brazilian was playing against his former team at the Stadio Olimpico for the first time since his January 2014 move to San Siro, and he equalised Antonio Candreva's early opener with a 24th-minute free-kick - for which Lazio centre-back Mauricio was sent off for a last-man challenge on Rodrigo Palacio.
The hosts battled superbly with 10 men, but their task was made even tougher when goalkeeper Federico Marchetti saw red for hauling down Mauro Icardi just after the hour.
Argentina international Icardi then fluffed his lines from 12 yards, but that miss did not prove decisive as Hernanes kept his cool when one-on-one with substitute goalkeeper Etrit Berisha six minutes from time to break Lazio's brave resistance.
Defeat leaves Lazio just three points above fourth-placed Napoli in the race for the Champions League, while Inter are three points off fifth and a guaranteed Europa League spot.
It took just eight minutes for Lazio to take the lead after a strong start.
Felipe Anderson beat Gary Medel on the left of the penalty area and his cutback was side-footed home by Candreva, with Samir Handanovic failing to get a strong enough hand on the ball.
Inter wasted a glorious chance to level as Icardi blazed over the crossbar when free on the right of the area.
Shortly after, Handanovic saved smartly from Marco Parolo's 10-yard effort, before the midfielder's follow-up header was nodded off the line brilliantly by Andrea Ranocchia.
The game turned in Inter's favour after Mauricio tugged Palacio to the ground when he chased a ball over the top. Referee Davide Massa deemed it a clear goalscoring opportunity and brandished the red card.
Lazio's misery deepened when Hernanes curled the resulting free-kick low into the bottom left-hand corner to silence a crowd that once cheered his goals.
Unsurprisingly Inter started the brighter after the break, although Miroslav Klose was denied by the legs of Handanovic when through one-on-one.
But another big decision went against the hosts in the 61st minute when Marchetti saw red for bringing down Icardi.
Icardi picked himself up, but his resulting spot-kick was saved low to the right by substitute goalkeeper Berisha.
There was one last twist, though, as Hernanes raced onto Mateo Kovacic's throughball and calmly slotted beyond Berisha in the 84th minute.