SkyTeam Alliance member China Southern has a one-stop San Francisco to Singapore for $510 round trip via Guangzhou, or even lower at $469-$477, if you take a longer two-stop flight with an overnight layover and transit through Wuhan too.
Lowest fares under $500 are for departures in August and September 2017. $510 fares are available August to November.
China Southern Airlines
San Francisco SFO – Guangzhou CAN – Singapore SIN – Guangzhou CAN – SFO
$510.04 round trip Orbitz ticket Thu Sep 7 – Tue Sep 19
China Southern Airlines
San Francisco SFO – Guangzhou CAN (10.5 hr overnight layover) – Singapore SIN – Guangzhou CAN – SFO
OmegaFlightStore.com ticket $469.09 round trip
