UV-range photoluminescence (PL) of free excitons was acquired in ultralow-nitrogen high-pressure, high-temperature diamond by mid-infrared (MIR, 4.0 and 4.7 μm) femtosecond (fs) laser pulses, indicating the direct interband photoexcitation. At lower laser intensities (< 10 TW/cm2) the excitonic PL yields exhibit highly non-linear dependences with Iλ2-scaling and power slopes N=17 (4.0 μm) and 14 (4.7 μm), still insufficient to cross over the direct bandgap >7.5 eV. At the Keldysh parameter value γ<1 such incomplete multi-photon interband excitation anticipates the hybrid “multiphoton + tunneling” photoexcitation generally predicted by the Keldysh theory, but never unambiguously experimentally demonstrated. At the intermediate values of the Keldysh parameter γ<1 the hybrid photoexcitation process is energy-scalable regarding intragap defect electronic states (> 3.5 eV), being envisioned by dislocation-related A-band (deep donor-acceptor traps) photoluminescence. These findings demonstrate the hybrid “multiphoton+tunneling” photoexcitation of intragap defect electronic states and interband photogeneration of free carriers in diamond in the intense MIR fs-laser fields governed by Iλ2-factor, and pave the way to defect/impurity band engineering of intragap non-linear optical properties in bulk dielectrics for their precise fs-laser nanomodification.