If you are taking the Singapore airline flight that leaves San Francisco at 8:15 PM and lands in Singapore at 5:45 AM, I wouldn’t worry about the transfer at all. If for some reason you can’t make that flight, there are a ton of flights throughout the day that will get you from SIN to BKK and Singapore Airlines would just put you on one of those.
However, if you are going to take the Singapore airline flight that leaves San Francisco at 9:40 AM and lands in Singapore at 7 PM in the evening, that flight could create more difficulties if you are running late because you may be delayed until the next morning for a flight to Bangkok. I think that it is unlikely that you are looking at taking this flight, since you have to get to SFO from DEN.
If you misconnect because of the fault of the airline, they will put you on the next flight. Sometimes they will also put you up in a hotel if it is an overnight delay but I would not count on that.