Return to the court after a hiatus of two months, Saina comfortably saw off the World number 28 Japanese 21-15 21-14 in a 20-minute women's singles second round contest.
The 22-year-old India will next take on the winner of another pre-quarterfinal match between qualifier Lok Yan Poon and fifth seed Tine Baun tomorrow.
Saina took time to get in her elements but once in her zone, she didn't delay the inevitable. She was lagging behind in the first game initially but soon gathered her composure to make it 1-0.
In the second game, the Indian opened up a 4-0 lead and extended it to 11-4 in a jiffy. Mitani did well to reel off six straight points and move to 10-11. But after the break, Saina crafted her way to seal the issue.
Saina recorded 27 smash winners in comparison to the 19 by her opponent. The Indian, who has beaten Mitani twice before, was also better at the nets.
Earlier, rising Indian shuttler Sourabh Varma stunned world number eight and home favourite Peter Hoeg Gade in straight games to enter the second round of the men's singles competition.
The 19-year-old Indian prevailed 21-18 21-14 against the sixth-seeded former All England champion in a contest lasting 42 minutes.
Sourabh, who was knocked out in the first round of the recent National Championships in Srinagar, did not get overawed by his five-time European champion opponent and matched his experience quite well on the court.
With 15 smash winners in the course of the match, Sourabh displayed quite a lot of grit against the 35-year-old crowd favourite.
Another India Ajay Jayaram, however, could not come up with a giant-killing effort and lost 21-19 8-21 9-21 against seventh-seeded Chinese Pengyu Du.
In the 59-minute match, Ajay, who was a finalist at the National Championships, showed promise to start with but eventually fizzled out against his fancied rival.