A semiclassical model of high-order harmonic generation (HHG) is developed and applied for the investigationof HHG in relativistic-intensity laser fields. A shift of photo-electrons along the wave vector of the pump wave is shown toreduce the harmonic intensity. In the low-frequency range ofthe spectrum of high-order harmonics, this intensity loweringis more noticeable than in the high-frequency range. Wepropose to use a standing wave instead of a travelling wave toincrease the HHG efficiency by reducing the longitudinal shiftof photoelectrons. The efficiency of using atoms in a standingwave is shown to decrease with the growth in the pump intensity. However, in the case of a travelling wave, this decreasein the HHG efficiency is even more noticeable. The use of astanding wave provides a considerable increase in the HHGefficiency, which may be as high as ~ 102 when pump radiation has an intensity I ≈ 1018 W cm-2 and a wavelength of0.3 μm.