The Japanese top seed, who reached the quarter-finals of the Australian Open in January, gained a key early break in the third set and finally secured the 3-6, 6-3, 6-4 win on his fourth match point.
"It was a tight game, but I'm really happy to win," said the 25-year-old. "I think it's important to win something like this sometimes, it's good for confidence."
Nevertheless, Nishikori said he'll have to play better in the quarter-finals, where he'll face another American qualifier, Austin Krajicek.
Krajicek, who survived two match points in a first-round win over Mikhail Kukushkin, toppled fifth-seeded Croatian Ivo Karlovic 7-6 (7/4), 4-6, 6-4.
Meanwhile, Third-seeded American John Isner pounded 22 aces in a 6-4, 7-6 (7/5) victory over Croatian Ivan Dodig and next faces compatriot Sam Querrey, who needed only 57 minutes to get past Jared Donaldson 6-4, 6-1.
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