Milan won 2-1 thanks to Tonali's injury-time goal.
“It was a wonderful feeling, one that brought us the result we worked so hard for after a bad start," Tonali told DAZN.
“Feeling most of the stadium was Rossonero was just crazy, we thank them for coming, it was like playing at San Siro and we showed that in the second half.
“I cannot say if it's one of the best moments of my life, but it is certainly one I will never forget. Scoring any goal for Milan is an unforgettable feeling, let alone one like this.
“We started badly in the derby too and had the strength to react, but without managing to score. After Lazio had that goal and one other chance, it turned into a siege and we didn't allow them anything.
“What we've lacked in the last few games was finishing those chances, but we managed it tonight."
The assist came from Zlatan Ibrahimovic, who knocked it down for Tonali's tap-in.
“It is rare to have a player like him in your team, so we need to make the most of him. It was his knockdown that changed the game this evening. From Ibra to the younger players like me, we all need to just keep going."