We show uniqueness for overdetermined elliptic problems defined on topological disks Ω with C 2 boundary, i.e., positive solutions u to ∆u + f (u) = 0 in Ω ⊂ (M 2 , g) so that u = 0 and ∂ u ∂ η = cte along ∂ Ω, η the unit outward normal along ∂ Ω under the assumption of the existence of a candidate family. To do so, we adapt the Gálvez-Mira generalized Hopf-type Theorem [19] to the realm of overdetermined elliptic problem.Whenfor any x ∈ R * + , we construct such candidate family considering rotationally symmetric solutions. This proves the Berestycki-Caffarelli-Nirenberg conjecture in S 2 for this choice of f . More precisely, this shows that if u is a positive solution to ∆u + f (u) = 0 on a topological disk Ω ⊂ S 2 with C 2 boundary so that u = 0 and ∂ u ∂ η = cte along ∂ Ω, then Ω must be a geodesic disk and u is rotationally symmetric. In particular, this gives a positive answer to the Schiffer conjecture D (cf. [33,35]) for the first Dirichlet eigenvalue and classifies simply-connected harmonic domains (cf. [28], also called Serrin Problem) in S 2 . MSC 2010: 35Nxx; 53Cxx.