The electronic spin susceptibility of YBa2Cu3O6.95 has been measured with h
igh precision up to 24 T with O-17 nuclear magnetic resonance. Its temperat
ure dependence can be accounted for by superconducting fluctuations that re
sult in a smooth crossover from the normal to the vortex liquid state. A ma
gnetic-field-temperature phase diagram for this crossover has been establis
hed having strong upward curvature. [S0163-1829(99)06033-6].