For the second match in succession the defending champion was forced to come from a set down to claim a 4-6, 6-4, 6-4 victory.
Having squeaked past qualifier Ryan Harrison in the second round, Nishikori would have hoped to have had a better time of it against Krajicek. However, the 24-year-old Krajieck proved an even harder task.
Krajicek claimed the lone break in the opening set as Nishikori struggled to get his serve going, finding the target with just 53 percent of his first serves and winning just 53 percent of the points when he did.
The world number five was able to tidy up these numbers in the second set and it paid dividends as he saved the lone break point Krajicek created and was able to convert one of the two break point opportunities that came his way.
Nishikori continued to up his game heading into the decider and got the early break he would have desired, but Krajicek was not going to go down without a fight and broke back. However, at 4-4 Nishikori once again broke the American, providing him the opportunity to serve out the match which he duly did.
Earlier in the day second see Kevin Anderson eased into last four, beating sixth seed Steve Johnson 6-4, 6-4. The big-serving South African needed just over an hour to wrap up the encounter, winning an impressive 32 of 36 points on his serve.
Much to the delight of the home fans, American Donald Young won through to the semi-finals, downing Australian Bernard Tomic 7-6 (8), 4-6, 7-5 after overcoming a mid-match wobble in which his serving went somewhat awry.