Birthday boy Nadal strolls into quarterfinals
Nadal is seeking to become the first man to win eight titles at the same Grand Slam event. He won a record seventh French Open championship last year and is 56-1 at Roland Garros.
Seeded No. 3, Nadal trailed early in each of his first three matches. But he played from ahead against Nishikori, the first Japanese man to reach the fourth round at Roland Garros since 1938.
Since returning in February from a seven-month layoff because of a left knee injury, Nadal is 40-2.
His opponent Wednesday will be the winner of the match between No. 7-seeded Richard Gasquet and No. 9 Stanislas Wawrinka.