Non-linear photoionization of molecules in the 10-50 eV range is a prerequisite for pump-probe measurements with sub-femtosecond resolution, but hitherto has been limited to femtosecond resolution, low repetition rate and high photon flux laser systems. We demonstrate two-photon single ionization of helium atoms using 100 pJ, 1.34 fs pulses (main peak FWHM = 680 as) at 1 kHz repetition rate with a central photon energy of 19.6 eV. We obtained an exponent of 2.27 +/- 0.21 for the intensity dependence of the signal and a two-photon ionization cross-section of 5.0 +/- 0.5 x 10(-50) cm(4) s. Our work opens the possibility of attosecond pump-probe measurements of ultrafast molecular processes. (C) 2017 Published by Elsevier B.V.