Iago Falque arrived at the back post to turn in M'Baye Niang's cross at the back post and put Genoa in front 17 minutes into a Serie A game that should have been played on Saturday, only to be postponed due to torrential rain.
Genoa, the away side playing on a ground they share with their big rivals, were eyeing a third consecutive win, but stayed in front for just two minutes as Roncaglia's poor touch gifted possession to Eder and the Brazilian's measured finish found the far corner of the net.
Stefano Okaka should have put out-of-sorts Samp in front after 26 minutes, but somehow fired over the crossbar from close range with the goal gaping.
Genoa almost won it right at the death when Juraj Kucka headed against the crossbar in the final minute of stoppage time and Andrea Bertolacci was denied by Emiliano Viviano following up before Kucka fired the loose ball over the crossbar in an almighty scramble.
Samp are now without a win in six games and remain level on points with Genoa.
