The influence of the hydrostatic pressure P up to 0.95 GPa on the excess conductivity sigma'(T) and the pseudogap Delta*(T) in optimally doped YBa2Cu3O7-delta single crystals (T-c similar or equal to 91.1 K at ambient pressure) is investigated by electrical resistivity measurements. A pronounced enhancement of the pseudogap under pressure of dln Delta*/dP approximate to 0.32, which is only a factor of 1.12 smaller than in slightly doped single crystals, is revealed for the first time. This implies a somewhat more moderate increase of the coupling strength in optimally doped cuprates with increasing pressure. Simultaneously, the ratio 2 Delta*(T-c)/k(B)T(C) approximate to 5 at P = 0 GPa, which is typical for high-temperature superconductors with strong coupling, increases by 16% with increasing pressure. At the same time, the pressure effect on T-C is minor: dT(C)/dP approximate to +0.73 K GPa(-1), whereas dln rho/dP approximate to (-17 +/- 0.2)% GPa(-1) is comparable with that in lightly doped YBCO single crystals. This suggests that the mechanisms of the pressure effect on rho(T) and T-C are noticeably different. Independently of pressure, near T-C, sigma'(T) is well described by the Aslamazov-Larkin (3D-AL) and 2D Hikami-Larkin fluctuation theories, exhibiting a 3D-2D crossover with increasing temperature. However, the temperature interval T-C < T < T-01, in which sigma'(T) obeys the classical fluctuation theories, is exceptionally narrow (approximate to 1.16 K). Nevertheless, a peculiarity at the temperature T-01, up to which the wave function phase stiffness in the superconductor is maintained, is clearly observed in the dependence Delta*(T). Below T-01 a fast growth of Delta*(T) is revealed for the first time. It can be associated with a sudden increase of the superfluid density, n(s), that is the density of fluctuating Cooper pairs (short-range phase correlations) forming in the sample when T approaches T-C. (C) 2016 Elsevier B.V. All rights reserved.