We prove a convergence theorem for a sequence of anti-self-dual connections on a family of products of two Riemann surfaces, where the metric of one factor shrinks, establishing the conjecture of Bershadsky et al. [Topological reduction of 4D SYM to 2D sigma-models," Nucl. Phys. B 448, 166 (1995)]. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3318164]