When we calculate rate of reaction pfor a complex reaction, if the rate determining step have intermediates, we calculate the rate of reaction using the step which is at equilibrium (and doesn't have any intermediate).
For example:-
Let the given reaction be a complex reaction$$\ce {A +B->C}$$ If this reaction takes place as follows:- $$\ce{A +B<=> P + Q}.....(fast)$$ $$\ce{P +Q-> C}.......(slow)$$ So here rate determining step is second one. But as it consists of intermediate we say that the first elementary reaction is at equilibrium and solve.
But why do we assume that the first reaction is at equilibrium? Maybe it is still not at equilibrium. Also what if they can never reach equilibrium because of the conditions? Then how can we calculate rate of reaction?