Top seed Akane Yamaguchi battled back to reach the Badminton Asia Championships final with a controversial win over Indian rival P.V. Sindhu on Saturday.
Japan's Yamaguchi will be the favourite to retain her women's singles crown when she faces China's unseeded Wang Zhiyi in the title decider on Sunday in Manila.
Fourth-seeded Sindhu claimed the first game of her semi-final against the reigning world champion 21-13 and was leading 14-11 in the second when the umpire ruled that the Indian player was taking too long to serve.
A clearly unhappy Sindhu, an Olympic silver and bronze medallist, argued her case with the officials but to no avail and she conceded a point penalty to her opponent.
She appeared to lose her focus and rhythm after that, allowing Yamaguchi to come back to claim the second game 21-19 and the deciding third 21-16.
"That was one of the reasons I lost, that's my feeling. At that point it was 14-11 and you never know, it would have been 15-11," Sindhu said of the umpiring decision which proved to be a turning point in the match.
"It was very unfair, maybe I would have won the match and I would have played in the final."
Wang, the world number 16, booked her place in the final with a shock 10-21, 21-12, 21-16 victory over South Korea's second seed An Se-young.
The men's final will be between Malaysia's Lee Zii Jia and Jonatan Christie of Indonesia.
Third-seeded Lee swatted aside China's unseeded Weng Hongyang in ruthless fashion with a 21-11, 21-19 win in 39 minutes.
In contrast, fourth seed Christie was pushed to three games by his fellow Indonesian Chico Aura Dwi Wardoyo before prevailing 21-9, 18-21, 21-16 in just under an hour.