The injury-prone Spaniard fell to the ground in pain after an awkward stumble in the first set, with several tense minutes passing before she gingerly returned to the court.

Marin went on to defeat the unseeded Pai 21-11, 18-21, 21-17 but it was an unsettling experience for the top seed, who nearly missed the world championships entirely due to a separate injury.

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"I just tried to not think about my ankle, because I was so scared, because maybe I could not play," she told reporters after the match.

"I have a physiotherapist here, so he is going to treat me now."

It was a case of history repeating itself for the defending champion, who sustained an injury to the same ankle in last year's world championships in Copenhagen, which she went on to win.

Elsewhere, men's number four seed Kento Momota of Japan breezed into the quarter finals with a straightforward 21-15, 21-16 win over Vietnam's Nguyen Tien Minh.

But the 20-year-old shuttler's campaign for a maiden world crown is about to hit a major hurdle, with a quarter final showdown against world number one Chen Long almost inevitable.

Denmark's mixed doubles pair Joachim Fischer Nielsen and Christinna Pedersen suffered a dramatic loss to Indonesia's Praveen Jordan and Debby Susanto, with the fifth seeds falling 22-20, 19-21, 23-21 to their lesser ranked opponents.

Chen, number-two seed Jan O. Jorgensen, five-time world champion Lin Dan and Malaysian superstar Lee Chong Wei are among the bigger names fighting for a quarter final slot later Thursday.(++++)

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