Single isolated attosecond (as) extreme ultraviolet (XUV) pulses can be generated via the nonlinear optical interaction between intense few-cycle laser fields and noble gas atoms. However, limited by today′s ultrashort pulsed laser technology, currently the shortest pulse duration of the few-cycle optical pulses is about two cycles, which is still not sufficiently short for generating single sub-100 as XUV pulses. Therefore, we propose to superimpose a relative-phase-controlled weak second-harmonic wave on the intense few-cycle pulse (e.g. 6fs) by which a single 65 as XUV pulse can be directly generated without any phase compensation process. Further, if the phase dispersion can be properly compensated, a single 23 as XUV pulse can be produced, which is below one atomic time unit (24 as).