Osaka, who won the Pan Pacific Open in Japan last month, converted five of eight break points and won the final 10 games in a row to wrap up the contest in 73 minutes -- her seventh consecutive win in the Asian swing.
"Today I was kind of happy because the first set was really tight," Osaka said. "I was thinking that it's been a while since I've kind of had to be really serious.
"It seemed like she was the type that would use my pace, especially on the forehand. I just tried not to give her too much."
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Riske started the brighter of the two in the last-16 encounter, taking a 4-2 lead in the opening set, but Osaka broke back twice before the American was bagelled in less than half an hour in the second set.
Canada's fifth-seeded Andreescu cruised past American qualifier Jennifer Brady with a 6-1 6-3 win. Brady faced 11 break points, of which four were converted by the Canadian who advanced in 67 minutes.
Defending champion Caroline Wozniacki also went through to the quarter-finals in straight sets with a 7-5 6-4 win over Czech Republic's Katerina Siniakova in a match where neither player was able to dominate their service games.
Wozniacki won seven of 10 break points while Siniakova converted five. The Czech saved three match points but 43 unforced errors tilted the tie in Wozniacki's favour.
Wozniacki will now face Russia's Daria Kasatkina, who beat compatriot Ekaterina Alexandrova 6-4 6-3.
US Open
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