On Thursday, the reigning champion survived a tough task against countrywoman Busanan Ongbamrungphan, wining their match 23-21 21-18 in 47 minutes. Marin beat Nitchaon Jindapol 21-14 21-18.
"It was a close match between me and Busanan as we know each other well. She put up a great fight but I took a better control in the end," said Ratchanok.
Ratchanok trails the Spaniard 1-2 in the head-to-head statistics. She made her greatest comeback by sweeping 10 points in a row from 11-18 in the final game of their All England Open quarter-final last month to beat Marin 21-18.
Red hot Dechapol Puavaranukroh reserved two quarter-final spots after his wins in the men’s doubles and mixed doubles.
Pairing with Kittinupong Kerden, they upset eighth seed Kim Astrup and Anders Skaarup Rasmussen of Denmark 21-19 20-22 21-18 in the men’s doubles second round. He then teamed up with Sapsiree Taerattanachai in the mixed doubles to oust seventh seed Chris Adcock and Gabrielle Adcock of Great Britain 21-16 21-19.
The other Thai duo moving forward was Bodin Isara and Nipitphon Phuangphuapet following their 21-11 and 21-14 win over Lu Ching Yao and Yang Po Han of Taiwan.