On the way there ...

Flying first to SIN means you have access to the nice NH F lounge at NRT.
Flying business to JNB means you have access to the SQ C lounges at SIN. UA arrives at T3, and IIRC JNB flight departs from T2. You can transfer between terminals airside - either by train or by walking (via T1).

On the way back ...

Flying first from SIN means access to the SQ F lounges. Even with terminal transfer you will still have time for a shower and breakfast in the lounge.