Please don't bring your Japanese client to Koh Grill. It's a fun place and Chef Patrick is very friendly but the food there is tweaked for Singaporean palates. It is not a place for subtle flavours.
If budget is no constraint, I personally like:
Ashino @ Chijmes
Shinji by Kanesaka
For something for more casual, but still good quality, I recommend:
Suju Japanese Restaurant
Something more modern:
High end Yakitori Bar:
Shirokane tori tama
Himonoya (not high end, but a fun place I like)
Keep in mind that with the exception of Wahiro and Suju, dinner at the places listed are mostly in the 300+/pax (or more) ballpark (before drinks), though lunch is typically a lot cheaper.
That said, I do agree with other suggestions here to bring your client to a non-Japanese place.
+1 for Ashino/Shinji, which are also the only ones I’ve tried.
Damn I gotta try the rest of your recs.
They reopened at Mohamed Sultan Road
Holy shit I am out of the loop!
Was still mourning it that day after dinner at Lawry's...
The new location is 2 doors down from Marco Pierre White's restaurant, and a stone's throw away from Esora.