Kunlavut Vitidsarn and Ratchanok Intanon booked their berths in the last 16 of the US$420,000 (approximately 17.4 million baht) BWF Toyota Thailand 2024 at Nimibutr Stadium on Wednesday.
However, the World Tour Super 500 event lost its men's top seed when Anders Antonsen suffered a shock exit in his first round match.
Defending champion Kunlavut, the eighth seed, took just over an hour to defeat Rasmus Gemke of Denmark 21-18, 21-12.
The victory sets up a last 16 clash between Kunlavut and Leong Jun Hao of Malaysia. If Kunlavut successfully defends his title this week, the 23-year-old will become the first Thai singles player to win back-to-back titles.
Fourth seed Ratchanok, meanwhile, defeated Keisha Fatimah Azzahra of Azerbaijan 21-11, 21-7 in her women's singles opener.
Ratchanok will play Gao Fengjie in the last 16 after the Chinese star defeated Samiya Imad Farooqui of India 21-13, 21-13 in her opening round match on Wednesday.
Qualifier Pitchamon Opatniputh fought her way into the last 16 after edging Kaoru Sugiyama of Japan 21-10, 14-21, 21-19.
She will face fellow Thai Supanida Katethong in the last 16 after the sixth seed defeated Karupathevan Letshanaa of Malaysia 21-12, 21-10 on Wednesday.
Pornpawee Chochuwong also needed three games to overcome Lin Hsiang-ti of Taiwan 21-9, 14-21, 21-17.
Another women's singles hope Busanan Ongbamrungphan came through her opening match in just 37 minutes, defeating Aakarshi Kashyap of India 21-13, 21-8.
China's second seed Wang Zhiyi of China eased through with a 21-8, 21-12 win over Huang Yu-hsun of Taiwan.
In the day's biggest upset, Angus Ng Ka Long of Hong Kong eliminated top seed Antonsen of Denmark in a 21-18, 14-21, 21-15 thriller.
The Hong Kong player will face Zhao Junpeng of China in the last 16.
Earlier, men's doubles pair Pharanyu Kaosamaang and Worrapol Thongsa-nga progressed to the last 16 after they beat Chia Weijie and Liew Xun of Malaysia 21-19, 21-12.
Men's doubles top seeds Satwiksairaj Rankireddy and Chirag Shetty of India also advanced after they defeated Nur Mohd Azriyn Ayub Azriyn and Tan Wee Kiong of Malaysia 21-13, 21-13.
