The American second seed left the home crowd in Osaka disappointed by dispatching Doi in 59 minutes with a 6-0 6-4 victory.
Keys, ranked 32 in the world, made a dominant start to proceedings at the WTA International hard-court tournament by condemning Doi to a bagel in the first set.
Doi displayed considerable signs of resistance in the second by breaking Keys twice, however, the 19-year-old's quality shone through as she progressed to set up a last-eight clash with Thailand's Luksika Kumkhum.
Kumkhum booked her place in the quarters by mounting an impressive comeback to end the hopes of Japan's Eri Hozumi with a 2-6 6-4 6-4 success.
Third seed Elina Sviotlina survived a scare as she also came from a set down to beat Japanese wildcard Naomi Osaka 3-6 6-3 6-4.
The Ukrainian world number 31 finally settled the contest in her favour after two hours and four minutes by winning a deciding set that featured five breaks of serve to finally end a gutsy challenge from the world number 265.
Elsewhere in the draw, Britain's sixth seed and 2012 champion Heather Watson was knocked out by Kazakhstan's Yulia Putintseva, who prevailed 6-4 7-5.