Neymar's corner was cleared forward by Marouane Fellaini, and picked up by Lukaku.
Lukaku found himself with space to turn, and launched himself on a counter down the pitch.
He ran nearly half the length of the pitch, with Brazilian defenders nowhere to be seen, before rolling a pass to De Bruyne on the right-hand side.
De Bruyne was given yards of space and launched a low screamer into the opposite corner of the goal.
Alisson in the Brazil goal was left a mere spectator as Belgium put themselves in the driving seat.