Lazio inflicted a third straight 'Derby della Capitale' defeat on Roma to hold on to bragging rights in the Eternal City, Stefano Mauri's goal proving key as both sides had a man sent off.
Erik Lamela temporarily lifted the Giallorossi above their fiercest rivals in the league table, scoring his seventh goal in six games with a confident header at the Stadio Olimpico.
But Vladimir Petkovic's Lazio seized control on a waterlogged pitch, Antonio Candreva and Miroslav Klose notching to turn the tables on Roma before half-time.
Daniele De Rossi conspired to get himself sent off in first-half stoppage time and Mauri clinched all three points with a close-range finish after the restart.
It proved to be a nervous finale for the Biancocelesti, though, as Mauri's dismissal allowed Miralem Pjanic to reduce the deficit from a bizarre free-kick.
A heavy thunderstorm provided the perfect backdrop to such a dramatic occasion, and early play was suspended as a floodlight failed.
Roma fans roared after the restart, as Lamela rattled home a corner with a diving header in the ninth minute.
Players on both sides struggled to play through driving rain and deep puddles, with Francesco Totti only able to test Lazio goalkeeper Federico Marchetti from distance.
Marchetti's opposite number Mauro Goicoechea got down to deny Klose a bullet header before resisting Abdoullay Konko from close range.
But the Uruguayan stopper fell foul of the wet weather in the 35th minute, letting Candreva's fierce free-kick slip between his gloves and into the net.
Klose smelled blood and rifled an effort over the bar before giving Lazio the lead just before half-time.
Hernanes' low shot was thwarted by a puddle, allowing the German striker to take advantage of the bemused Giallorossi backline and prod the ball home.
Four extra minutes also proved fruitful for the Aquilotti as De Rossi found the time he needed to strike Mauri in the face and receive the red card.
Things went from bad to worse for Zdenek Zeman's men after the interval as Mauri stole on to a poor backpass from Ivan Piris to beat Goicoechea.
Roma regrouped, though, and Pablo Osvaldo was desperately unlucky not to reach Totti's excellent low cross as the rain continued to pour down unabated.
Marquinho tried his luck left-footed, his dangerous effort drifting over the bar, but Pjanic would have better luck from distance four minutes from time.
Mauri could have no complaints when he picked up a second yellow card - and his marching orders - for a careless handball in Roma's half.
Pjanic peered through the rain to identify the red shirts up front, but instead his drifting free-kick wrongfooted Marchetti and glided into the net.
Zeman frantically issued messages for his side to go all out for the equaliser, but Osvaldo made a hash of Michael Bradley's sumptuous through-ball and the derby was lost.