JAKARTA (Reuters) - India's P.V. Sindhu caused the first major upset at the badminton World Championships when the 20-year-old stunned Olympic champion Li Xuerui of China 21-17, 14-21, 21-17 in a pulsating women's singles third-round encounter on Thursday.

Sindhu often saves her best for this tournament and will be looking to improve on her two previous bronze-medal performances if she can maintain the form that saw her storm to victory past the third seed in 50 minutes of action in Jakarta.

There was almost another upset in same half of the draw when top seed and defending champion Carolina Marin dropped the second game of her match against Taiwan's Pai Yu-po. However, the injury-prone Spaniard dug deep to advance 21-11, 18-21, 21-17.

Second seed Saina Nehwal overcame a sluggish start against Japanese southpaw Sayaka Takahashi to book her place in the last eight with a gutsy 21-18, 21-14 victory over her fast-starting opponent.

Nehwal looked completely out of sorts in the opening exchanges as she struggled with the drift inside the Istora Senayan arena but the former No. 1 knuckled down to rally for victory after trailing 11-3 at the first mid-game interval.

In the men's draw, top seed and defending champion Chen Long put on a fine display of attacking badminton and supreme court coverage with a 21-17, 21-18 victory over unseeded Thai Tanongsak Saensomboonsuk to book his place in the quarter-finals.

Men's fourth seed Kento Momota of Japan also advanced with a straightforward 21-15, 21-16 win over Vietnam's Nguyen Tien Minh.