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The influence of the hydrostatic pressure P up to 0.95 GPa on the excessconductivity and the pseudogap {\Delta}*(T) in optimally doped YBCO singlecrystals (T_c = 91.1K at ambient pressure) is investigated by electricalresistivity measurements. A pronounced enhancement of the pseudogap underpressure of dln{\Delta}*/dP= 0.32, which is only a factor of 1.12 smaller thanin slightly doped single crystals, is revealed for the first time. This impliesa somewhat more moderate increase of the coupling strength in optimally dopedcuprates with increasing pressure. Simultaneously, the ratio2{\Delta}*(T_c)/k_BT_c=5 at P=0 GPa, which is typical for high-temperaturesuperconductors with strong coupling, increases by 16% with increasingpressure. At the same time, the pressure effect on T_c is minor: dT_c/dP=+0.73K/GPa, whereas dln{\rho}/dP=-17% 1/GPa is comparable with that in lightly dopedYBCO single crystals. This suggests that the mechanisms of the pressure effecton {\rho}(T) and T_c are noticeably different. Independently of pressure, nearT_c, the excess conductivity is well described by the Aslamazov-Larkin (3D-AL)and 2D Hikami-Larkin fluctuation theories, exhibiting a 3D-2D crossover withincreasing temperature. However, the temperature interval T_c<T<T_{01}, inwhich excess conductivity obeys the classical fluctuation theories, isexceptionally narrow (~1.16 K). Nevertheless, a peculiarity at the temperatureT_{01}, up to which the wave function phase stiffness in the superconductor ismaintained, is clearly observed in the dependence {\Delta}*(T). Below T_{01} afast growth of {\Delta}*(T) is revealed for the first time. It can beassociated with a sudden increase of the superfluid density, n_s, that is thedensity of fluctuating Cooper pairs (short-range phase correlations) forming inthe sample when T approaches T_c.

Peculiarities in the pseudogap behavior in optimally doped YBa2Cu3O7-{\delta} single crystals under pressure up to 1GPa