World No. 6 Saina, who struggled with injuries this season, fought for 68 minutes before her Japanese rival got better of her.
Saina has been clubbed in Group B along with world No 1 and Olympic champion Li Xuerui of China, world No 7 Yeon Ju Bae of Korea, besides world No 13 Mitani during the December 11-15 year-end tournament, which is being fought among top eight performers from the 12-event Super Series circuit.
Saina hardly had to break sweat in the opening game today. From being tied at 7-7, the Hyderabad-based shuttler opened a 12-7 lead and eventually wrapped it up 21-19 in style.
Saina began the second game also with a bang as she straightaway opened up a 7-0 lead. But the Japanese reduced it to 5-7 before making it 11-11.
In the final game, Saina once again took an upper hand early on and always managed to maintain a slight lead but Mitani came from behind to make it 17-17.
At 19-19, Saina's poor fitness became a reason for her downfall as Mitani closed the match with couple of winners.