Aiko Yoshitomi triumphed in her first match of the All Japan Tennis Championships, a test event for next year's Olympic Games in Tokyo.
Taking place at Ariake Tennis Park, the competition is a chance for Tokyo 2020 organisers to test the facilities and procedures at the venue before the Games next year.
With an all-Japanese field competing, Aiko Yoshitomi defeated compatriot Misaki Matsuda 4-6, 7-5, 6-2 in the main draw of the women's competition due to conclude with the final on November 3.
Sixth seed Yuriko Miyazaki was then stunned by Shina Haruna, losing 3-6, 6-1, 6-1.
Another upset came in the clash between Eri Shimizu and Risa Ushijima.
Eighth seed Ushijima was beaten 6-0, 4-6, 7-5.
In the men's competition, 18-year-old Keigo Saito was victorious in his first match, overcoming Akira Oue 3-6, 6-1, 6-1.
The oldest participant crashed out of the tournament, however.
The 43-year-old Katsushi Fukuda lost 6-4, 6-4 to Akira Takeshima.
During a venue tour at the Tokyo 2020 World Press Briefing earlier this month, it was revealed that work on court two, the Park's third showpiece arena, has not started yet, with organisers confirming plans are on schedule for a March 2020 completion.
The park's impressive centre court - Ariake Colosseum - staged the final of the Rakuten Japan Open, when Novak Djokovic of Serbia claimed glory with a 6-3, 6-2 win against Australian John Millman.
The Ariake Colosseum is expected to home 10,000 spectators, with a retractable roof giving organisers an extra method to shelter fans from extreme temperatures, should the heatwave from July be repeated in nine months.