Sasao produced a battling display as she fought back from two games down to claim a 5-11, 9-11, 11-5, 11-7, 4-11, 12-10, 11-5 win at the Arena Asarel.

The Japanese player will take on compatriot Maki Shiomi in the next round after she beat Lee Yu-Peng of Chinese Taipei 11-9, 9-11, 11-2, 11-8, 12-10 to reach the last 16.

Top women's seed Kasumi Ishikawa is also safely through as she overcame Iveta Vacenovska of the Czech Republic 6-11, 11-5, 11-6, 12-10, 11-4.

Ishikawa will be involved in another all-Japanese affair as she will go up against Mizuki Morizono in the next round.

Morizono progressed with a 11-4, 11-7, 11-13, 11-7, 11-5 victory over home hope Maria Yovkova.

In the men's singles event, top seed Dmitrij Ovtcharov, the London 2012 singles bronze medallist who is ranked at number five in the world, booked his place in the last 16 as he proved too strong for Russian Kirill Skachkov.

Ovtcharov was made to work hard for his win as he dropped two games on the way to clinching a 11-7, 11-4, 7-11, 9-11, 15-13, 11-1 victory.

His opponent in the next round will be Elias Ranefur of Sweden, who progressed after French opponent Quentin Robinot withdrew.

Robinot had lost the opener 11-6 and was unable to continue, handing Ranefur a place in the round of 16.

Quadri Aruna of Nigeria, seeded third, joined Ovtcharov in securing his spot in the last 16 after overcoming Can Akkuzu of France 11-5, 8-11, 11-6, 11-6, 9-11, 6-11, 11-8.

Japan's Yuto Kizukuri, who beat Soumyajit Ghosh of India 11-4, 8-11, 11-4, 11-9, 12-14, 12-10 in his opener, will clash with Aruna tomorrow.

The tournament in Bulgaria continues tomorrow.