Troubled in his first two rounds where he had to come from a set back, Nadal celebrated his 27th birthday in style with a brutal demolition of 13th seed Nishikori.
Only in the first set did Nishikori, looking to become only the second Japanese man to reach the last eight in Paris, offer any sort of challenge to the defending champion.
Nadal broke serve with the aid of a fortunate net cord in the fifth game of the opener and that proved enough for him to take the first set.
Nishikori had a glimmer of a chance when he had a break point in the second game of set two but failed to convert it and was broken in the next game when he flailed a high backhand volley into the tramlines.
From then on Nadal was in total command and finished the match by hooking a forehand winner down the line after a little over two hours before waving to the crowd as they sang happy bithday to the the king of clay.
"To celebrate my birthday here on centre court in Paris is a very special moment for me," Nadal said courtside before a huge birthday cake was wheeled on to court.
"Today I played a bit better than in the previous days."
Third seed Nadal will face the winner of Stanislas Wawrinka against Frenchman Richard Gasquet in the quarter-finals.
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