How to model the intial value of a linear system using Simulink?

I have seen more than one examples presented in tutorials to show how
to model control system in Simulink, most of which reads as below:

r->sum->PID->system Transfer Fcn->y
       |                                               |
       -----------------------------------------------

But since transfer function assumes that the initial value in systems
should be zero, how  can I model a system with intial values together
with the transfer function in Simulink?

for example, I have a state-space model like below:

x' = A*x + B*u
y = Cu

assuming it is a SISO system, and  x(0) should be considered, after I
use the ss2tf() function to transform it into a transfer function,
then how can I include the x(0) in the Simulink? Considering a case
that there are lots of nonlinear parts in the whole system, a simpler
treatment using feedback() or lsim() can't work.

Thanks in advance!