I'v been to The Central (Clarke Quay) and I find that this Japanese restaurant by the river called The Sun serves very nice Japanese food, sashimi included.
There's another mall in CQ called Lot 1 that is full of authentic Japanese restaurants, you could give it a try. I would think the food there is authentic as the chefs are authentic, and there is a condo full of Jap expats living nearby. They frequent this mall and its restaurants.