A few years ago I did AKL-HKG-YYZ in one go in Economy, and HKG-YYZ is about half an hour shorter than HKG-JFK. I don't really advise it, but would make a few suggestions:
1, If you have access to lounges, shower to freshen up at every stop. Also, you'll get much better food in the lounge than you do in Economy.
2, Take something on board that is fairly mindless and you've meant to do for a long time (I deleted emails for a few hours over the Bering Strait). You'll soon have seen every movie you want to that is on the IFE. A book/magazines are not a bad choice either.
3, Assuming you are going into Manhattan, and it is not late at night, take public transport (the subway) into NYC. The subway journey gives you time to adjust.