We report an increase in pulse energy to 5.1 microJ obtained directly from a femtosecond diode-pumped Yb:YAG thin disk laser without external amplification. Stable passive mode locking was obtained with a semiconductor saturable absorber mirror (SESAM). The laser delivers 63 W of average output power in a nearly diffraction-limited beam (M2=1.1) at a center wavelength of 1030 nm. The pulse repetition rate is 12.3 MHz, and the pulses have a duration of 800 fs, which results in a peak power of 5.6 MW. The laser was operated in a box flooded with helium because the nonlinearity of air was found to be a limiting factor for the stability of the pulse formation at increasing pulse energies.