United Airlines has San Francisco to Singapore nonstop flights on UA 1 and UA 2 for $746 round trip. However, fly UA 1 or UA 2 in one direction and fly Cathay Pacific between Hong Kong and Singapore on either the outbound or inbound flight for an overnight, then the price drops to as low as $548-$560 round trip to fly United between SFO and Singapore and Hong Kong to SFO.
Personally, I think the better itinerary is book Hong Kong on the return flight. That is a harder ticket to find. A 14 hour economy flight between SFO and HKG will likely mean stretching out on a bed is the most pressing desire if you stopover on the outbound flight.
$548 to $560 fares are available on select dates from August to December 2019.
Mon Sep 9 San Francisco SFO – Hong Kong HKG United Airlines
Tue Sep 10 Arrive Hong Kong 6:45pm
Wed Sep 11 Depart Hong Kong for Singapore 11:15am Cathay Pacific
Wed Sep 11 Arrive Singapore 3:05pm
Tue Sep 17 Singapore SIN – San Francisco SFO United nonstop