Mate, if your route is SIN-Tokyo and you fly F, it is a no brainier to take SQ. I flew that once last year, ended up going through NRT to make a dinner. NRT is quite inconvenient but frankly this is far better than the craziness of connecting in Hong Kong to CX548, which is the only flight each day to Tokyo which guarantees F class. SQ has a second flight to HND with F too IIRC, but it lands too late to make a dinner. But that's another great option.
Everyone else above is giving spot on advice. You are likely to be on a regional config overnight to HKG. Dreadful. And thet connect F to Tokyo which in winter months can be barely 3+ hour flight flying time, not enough for a half decent nap. Food is J class breakfast. Etc. You couldn't pay me to do that routing!
As for the return, HND-HKG in F on CX is considerably better, but then you still have to connect to SIN on the red-eye? Or latest daytime flight? And who knows if CX flies a flat bed on that config. No I really cannot recommend this one bit!
And if you're flying J not F, still do SQ or JAL nonstop! No way do you want to fly CX for this itinerary. My two cents.