Originally Posted by
flyerby
Since you booked on UA, TG may not be inclined to help in case of a delay by SQ even on a single PNR. You are insured by UA and not TG. TG may play nice though. If TG doesn’t play nice do insist. It may help.
Under IATA obligations, if SQ is the late-delivering carrier, they're the ones to rebook the passenger to the final destination. This applies regardless of ticket stock.
If the delay is known before departure and breaches the MCT, they'll probably rebook the OP onto a direct SIN-BKK.