Here's what I did on my last trip to Fiji (same duration):

1. Daytime flight to NRT, no sleeping.
2. Delay and overnight hotel stay in NRT due to aircraft problems. Good night's sleep.
3. Flight to NAN during day, again no sleep.
4. Arrive at my hotel in Suva just before midnight. Good night's sleep after a light room service supper.
5. I found this accidental strategy so good that I took a three day break in LA on the way back.

In general, if you can afford the time (and especially if you're in Y). A day or two between these long haul flights works wonders. You'll be very lucky not to get some jet lag at the other end though (difficulty getting to sleep at the proper times, waking up at 4AM etc) but at least you'll feel reasonably ok when you reach your destination.

Regards
I