We report on top-gated indium-zinc-oxide (IZO) thin film transistors (TFTs) with an in-situ formed HfO2 gate dielectric insulator. Building on our previous demonstration of high-performance IZO TFTs with Al2O3/HfO2 gate dielectric, we now report on a one-step process, in which Hf is evaporated onto the 20 nm thick IZO channel, forming a partially oxidized HfOx layer, without any additional insulator in-between. After annealing in air at 300 degrees C, the in-situ reaction between partially oxidized Hf and IZO forms a high quality HfO2 gate insulator with a low interface trapped charge density N-TC similar to 2.3 x 10(11) cm(-2) and acceptably low gate leakage <3 x 10(-7) A/cm(2) at gate voltage V-G = 1V. The annealed TFTs with gate length L-G = 50 mu m have high mobility similar to 95 cm(2)/V.s (determined via the Y-function technique), high on/off ratio similar to 10(7), near-zero threshold voltage V-T = -0.02 V, and a subthreshold swing of 0.062 V/decade, near the theoretical limit. The on-current of our proof-of-concept TFTs is relatively low, but can be improved by reducing (L)G, indicating that high-performance top-gated HfO2-isolated IZO TFTs can be fabricated using a single-step in-situ dielectric formation approach. (C) 2016 Elsevier B.V. All rights reserved.