  We show that the 5D Nekrasov partition functions enjoy the enhanced global
symmetry of the UV fixed point. The fiber-base duality is responsible for the
global symmetry enhancement. For $SU(2)$ with $N_f\leq 7$ flavors the
fiber-base symmetry together with the manifest flavor $SO(2N_f)$ symmetry
generate the $E_{N_f+1}$ global symmetry, while in the higher rank case the
manifest global symmetry of the two dual theories related by the fiber-base
duality map generate the symmetry enhancement. The symmetry enhancement at the
level of the partition function is manifest once we chose an appropriate
reparametrization for the Coulomb moduli.
