Van Der Poel, 26 years old, covered the 178 kilometers between Neuhausen am Rheinfall and Lachen in 4:12.30 hours, one second faster than German Maximilian Schachmann (BORA-hansgrohe), second, and three for compatriot Wout Poels ( Bahrain-Victorious), third.
Küng remains the leader of the Tour of Switzerland.
The only Portuguese in the competition, Rui Costa (UAE Emirates), was 56th in the stage, 2.03 minutes behind the winner, and rose to 57th place overall, now 2.40 behind the leader, in a race he has already won by three times.
Today the third stage connects Lachen to Pfaffnau in 182.1 kilometers.