Lewis Hamilton topped the timesheets, but was just under two-tenths quicker than his nearest competitor Max Verstappen at the Marina Bay Street Circuit. Valtteri Bottas raced in FP2 despite his crash in the opening session earlier in the day.
The Finn wasn't able to replicate his colleague's speed in Asia, finishing in P4, almost a second off Hamilton's fastest time of 1:38.773 under the lights in Singapore.
"The pace of Mercedes, especially that of Hamilton, was rock-solid," di Resta said on Sky Sports F1. "It is clear that Mercedes stands head and shoulders above the rest in the early phase of the long-runs."
Verstappen finished with a best time of 1:38.957 and even topped the earlier session's timesheet for Red Bull, but di Resta believes Mercedes' speed is "head and shoulders" above their competitors.