Start working on your plan B & C is my initial response - how soon is your upcoming flight, OMG. Personally, I wouldn't do any domestic to int'l transfer at SFO with less than 90 minutes, even if it's UA to UA - and last time we connect thru it to HKG, we had 120 minutes ++ and flight was delayed inbound due to weather (fog !!!) and air traffic, and we ran like rabbits on fire to catch our connection as they're already boarding ...

Is this booked on a single ticket/itinerary or 2 different e-tickets, etc. ? Your UA flight SAN-SFO is on a regional jet so I assume that you will be checking luggage @SAN - all the way (interlines) to your final destination - as, otherwise, reclaiming them upon arrival at T1 and the clock will run out on you with the cutoff time for SQ checkin.

Unless I'm mistaken, SQ flight (int'l) will not depart from T3 but rather the International Terminal, next to T3 and you will need to go thru Security (TSA) as part of the check-in.

Shuttle flights like yours tend to park closer in to the main terminal instead of out in the far end. It is important to NOT exit the North Terminal (T1) where United Domestic is - follow the signs for International Terminal as there is a sterile walkway airside (by Gate 75) to concourse G or the International Terminal (thus, you will not have to re-clear security - if your luggage are tagged all the way thru SQ to your destination plus all Boarding Passes issued, etc.)

Flights coming from LAX (and SAN) are notarious and infamous for being delayed and held up at the departing airport, to give priority to long-haul and oversea inbound flights long on fuel - throw in typical SFO weather & fog that's unpredictable, you get the picture ....

See here - http://www.flyertalk.com/forum/unite...-thread-3.html

If you could, switch to an much earlier flight leaving SAN to allow extra cushion in delays and unexpected surprises while transiting thru SFO. Surely those expert SFO locals will fill in more details & clarify things further promptly.

Regardless, I would keep a close watch for flights betweeen SAN-SFO on the morning of your flight, if early ones begin to be delayed - expect it to get worst as these are typical shuttle flights that are quickly turned around for multiple daily trips. Hopefully, everything will work out the best for you ...