I don't know of a direct to SIN, I have always gone via HGK or NAR. Via NAR is same continuing on flt #, ususally they give you the same seat for both segments but two boarding passes

forgot to mention, on full or near full flts, the UA seat selector does not allow for seat changes or selection. Has to be done at the airport. I found CO's seat selection process to be much more user friendly. Hope they go with CO website model post merger