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Boulter is looking to finish a season where she has won two WTA titles with a flourish and has reached her first quarter-final since Eastbourne in June.
The clinical victory over Okamura sets up an intriguing clash with Canadian wildcard Bianca Andreescu.
Andreescu, 24, won the 2019 US Open as a teenager but has been plagued by injury problems since.
“Bianca is an incredible fighter, won a Grand Slam and is playing some good tennis,” said Boulter, who has no ranking points to defend and could move back into the top 30 this week.
“It’s a free swing for me at the end of the year and I have nothing to lose.”
Boulter is seeded ninth at the WTA 500 event in the Japanese capital, with China’s Zheng Qinwen, Russian Daria Kasatkina, Diana Shnaider and Canada’s Leylah Fernandez left in the draw as higher-ranked players.