Alberto Gilardino fired the visitors in front inside 20 minutes but Genoa suffered a setback two minutes before the break when Thomas Manfredini was dismissed for a second caution.
The visitors weathered the Cagliari storm for much of the second half before finally succumbing when Sau opened his account in the 77th minute, before the Italy international bagged the winner in the final minute.
Seventeen minutes had gone when Genoa, who came into the match on the back of a five-game unbeaten run, took the lead through Gilardino's smart finish from Francesco Lodi's assist.
Gilardino then had a golden chance to make it two just before the half-hour when he beat the offside trap but could only fire wastefully over.
Genoa's lead appeared under little threat but all that changed on 43 minutes when Manfredini, already on a yellow card, was dismissed after a foul on Daniele Conti.
The hosts looked to make the most of their numerical advantage after the restart with Davide Astori and Mauricio Pinilla both seeing headed efforts come close to levelling.
With 56 minutes gone Conti's volley was brilliantly tipped over by Mattia Perin, before Genoa briefly broke out from their own half as Luca Antonelli saw a shot from close range kept out by Vlada Avramov.
It took a moment of class from Sau to get Cagliari level, the diminutive striker drifting past two defenders before firing a powerful shot beyond Perin.
The Genoa goalkeeper looked as though he had secured a point for his side when he saved well from Conti's curling effort in the 84th minute, but there was still time for the hosts to win it as Nene's spectacular attempt fell to Sau and the striker did the rest from close range.