The Spaniard looked at home on his preferred surface, and he had enough to record a straight-sets victory against his higher-ranked opponent to set up a clash with Rafael Nadal.
Nishikori began the set on the front foot by securing an early break of serve with a number of powerful shots across the court, which punished Ramos's sluggish start.
Despite grabbing the first break, Nishikori stumbled by failing to hold his serve to give his Spanish opponent the advantage in the set to push on and snatch the lead.
Games were won on serve at the start of the second set, with both competitors forcing the other to the wire on a number of games, but neither could initially find the breakthrough.
Eventually, the second set went into a tie-break, and after taking the first mini-break, Ramos held his nerve to seal his passage through to the quarter-finals.