Li Xuerui hits a forehand return to P.V. Sindhu in their women's singles match at the BWF World Championships in Jakarta on Thursday. Photo: CFP

World No.1 Carolina Marin's defense of her world crown received a boost on Thursday, with key rival Li Xuerui of China crashing out of the world championships in Jakarta.

Li, an Olympic champion and silver medalist at last year's world championships, became the highest-ranked ­casualty of the tournament so far when she fell in a marathon contest to ­11th-seeded P.V. Sindhu 21-17, 14-21, 21-17.

The crestfallen third seed said she wouldn't dwell on her shock exit and pledged to support her Chinese compatriots still in contention for medals.

"I prepared to the maximum, so I'm not going to stay down for long," she said via a translator.

The Chinese shuttler was upstaged at last year's world championships by Marin, and badminton fans had been hoping for a grudge match between the two rivals before Li's sudden departure.

But in a day of mixed fortunes the Spaniard rolled her ankle during her third-round win over Chinese Taipei's Pai Yu Po. 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.

"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.

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.

In the men's competition, world No.1 Chen Long coasted into the quarter­finals, thrashing Thailand's Tanongsak Saensomboonsuk 21-12, 21-7.

That may be bad news for fourth seed Kento Momota, who could see his dreams of a maiden world crown crushed if he faces the defending Chinese champion as expected in the semis.

Five-time world champion Lin Dan defeated Denmark's Hans-Kristian ­Vittinghus 21-9, 21-13. Second seed Jan O. Jorgensen of Denmark beat South ­Korea's Son Wan Ho 21-19, 21-11. Malaysian superstar Lee Chong Wei also ­advanced to the last eight with a 21-17, 21-19 win over China's Wang Zhengming.

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.