The 33-year-old returned at the beginning of the season after a serious back problem destroyed his previous two campaigns and raised doubt about his future in the game.
His determination to play again was rewarded by two goals in Sunday's 2-1 win at Chievo, Milan's third straight victory in all competitions, which powered Leonardo's previously faltering side up to sixth.
"It's my first brace, it's beautiful," he told reporters. "I'm happy for how things are going, better than I had expected because before the injury I did not score goals.
"Now I go forward more because some old guys left," he joked, referring to the retired Paolo Maldini among others.
Milan, with Nesta also in good form in defence, have the chance to move further up the table and take advantage of second-placed Sampdoria, third-placed Juventus or both dropping points when the two meet in Turin on Wednesday.
But while former Italy defender Nesta is revelling in the moment, another Milan player is struggling to make an impact.
Netherlands striker Klaas Jan Huntelaar was Milan's big close-season buy but he has failed to nail down a first team place or score so far this term.
He was hauled off with Milan 1-0 down at Chievo and is unlikely to start the game at improving Napoli, who won 1-0 at Fiorentina on Sunday thanks to another late goal.
Inter Milan's Wesley Sneijder, another recent Dutch recruit to Serie A, is faring much better than Huntelaar but is all but out of the leaders' home game with Palermo on Thursday.
The playmaker, who is key to linking Inter's midfield with the attack, suffered a hamstring strain after scoring a free kick in Saturday's 2-1 win over Catania.
However, midfielder Thiago Motta and striker Diego Milito could return from injury on Thursday.
