Originally Posted by dhuey

Welcome to FT, theheroflyer! It helps a lot that you are coming in Terminal 3 on your United flight and can walk over to the G Concourse of the International Terminal for your Singapore flight. Important: do not exit security. Stay airside and follow the signs to the International Terminal, Gates G91-G102. Just past gate 75 and with security on your left, you'll see a hallway that takes you to International-G.

I would be comfortable with your two-hour layover, but of course there is always some risk if your flight to SFO is delayed. If you walk quickly, it should be about 5-15 minutes from gate to gate, depending on the gates. Good luck and safe travels.