This document discusses, among other things, systems and methods to discriminate between a ventricular tachyarrhythmia (VT) and a supraventricular tachyarrhythmia (SVT), such as upon detecting sudden onset and one-to-one tachycardia. In certain examples, a detected tachyarrhythmia is analyzed to determine whether it is sudden onset and 1:1. If so, a first fast beat is identified. One or more ventricular intervals in close proximity to the first fast beat are analyzed to determine an initial classification of either VT or SVT. The initial classification is used to adjust a morphological feature correlation coefficient (FCC) threshold. A morphology analysis is performed with the adjusted FCC threshold value to yield a secondary classification.