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[Display omitted] • We achieved ultrathin flexible AZO electrodes with low resistivities (less than100 μΩ•cm). • We demonstrated the mechanical robustness of ultrathin flexible AZO electrodes. • We discovered abnormally high carrier concentrations in strained epitaxial AZO. Aluminum-doped zinc oxide (AZO) thin films are popular transparent electrodes for optoelectronics and photovoltaics. However, next generation flexible devices demand not only high conductivities but also low thicknesses in AZO electrodes to stay robust against deformations. The two requirements contradict each other due to the scattering of carriers at grain boundaries in polycrystalline AZO electrodes. Herein, this work develops ultrathin, highly conductive, single-crystalline, and mechanically robust AZO electrodes on flexible Hastelloy substrates. The epitaxial growth of AZO on polycrystalline Hastelloy is empowered by an advanced buffer architecture topped with a biaxially-textured MgO layer. Thereby, a c -axis epitaxial strain is generated in the AZO electrodes and then regulated by varying the thickness of the electrodes. Abnormally high carrier concentrations are detected in the AZO electrodes likely caused by the piezopotential generated by the epitaxial strain. While the highest resistivity value of the epitaxial AZO electrodes is within the mainstream values of AZO electrodes on various substrates, a resistivity as low as ~22 μΩ•cm is demonstrated in the ultrathin electrode with a thickness of ~28 nm. The mechanical durability of the ultrathin AZO electrode is validated by bending tests of 1000 cycles. [ABSTRACT FROM AUTHOR]