Unseeded Williams, who is now ranked 62nd in the world after struggling with injuries for much of the last 12 months, cruised to a 6-3 6-1 win over her German opponent.
With her sister Serena withdrawing from the tournament due to fatigue, world number two Azarenka is the top seed and will not relish facing the older Williams, who has won both of her previous two encounters against the Belarusian - the last of which was in 2010.
Williams' American compatriot Sloane Stephens was also a comfortable winner in Japan.
The 20-year-old ninth seed was rarely stretched as she came through 6-3 6-2 against Switzerland's Stefanie Voegele.
Fellow seeds Samantha Stosur and Kirsten Flipkens were given tough examinations before booking their place in the second round.
Australian 12th seed Stosur, the 2011 US Open champion, bounced back from losing the first set to win 2-6 6-4 6-1 against Alize Cornet of France.
Fourteenth seed Flipkens was given a thorough test before the Belgian edged to a 7-6 (7/4) 7-5 over Czech opponent Barbora Zahlavova Strycov.
Fifteenth seed Sorana Cirstea was a 6-4 6-4 winner against Julia Goerges, while Dominika Cibulkova, seeded 16th, eased to a 6-3 6-3 victory over Urszula Radwanska.
Two-time grand slam winner Svetlana Kuznetsova beat Paula Ormaechea 6-1 6-2, while there were also wins for Maria-Teresa Torro-Flor, Lucie Safarova, Belinda Bencic and Elina Svitolina.