Please do not do that. The agent will have no idea what you’re talking about, and it doesn’t work the way you’ve inferred anyway. WineCountryUA’s statement, while true, is somewhat misleading in this case. The fares you’re seeing are correct, and not a result of anything SQ is or is not doing with regard to the fare table. In this case, a UA fare (not an SQ fare) is being used;, and UA has an arrangement in place with SQ to carry passengers to MAA.

The primary reason that you can’t get the lower price on a single ticket is that UA has written fare rules into its SFO-SIN fares that prohibit end-on-end combinations except for certian domestic UA flights. No agent can waive those rules, and you’d be wasting your time and the agent’s by calling.

If SQ is offering a better price on a single ticket, why not fly them?