World Championships: Top seed Yamaguchi suffers shock exit
Chen Yufei, the 19-year-old world junior champion, defeated the top seed 21-18, 21-19 in 52 minutes.
Akane Yamaguchi admitted she had been well below her best.
China's Chen Yufei announced herself as a new star of international badminton on Thursday when she knocked out Japan's top seed Akane Yamaguchi in the third round of the world championships.
The 19-year-old reigning world junior champion shocked the Japanese favourite with a straight games 21-18, 21-19 victory and now meets Thailand's Ratchanok Intanon in the quarterfinals.
In the second game, Chen clawed her way back from 11-14 behind, winning six points in a row and then claimed: "This is the greatest result of my career.
"It was one of my goals this year to beat Akane and now I have achieved it. I am very excited and so happy.
"In the middle of the second game I started to make some mistakes. But then I got myself together. I came to these Championships with several goals. This was the biggest one.
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"Now I am just going to take it day by day. I am really looking forward to the quarter-final."
Akane, the world number two, was desperately disappointed.
"I am very sad," she admitted. "I made too many errors but she played well."
In the men's singles, top seed Son Wan Ho needed three games to overcome Thailand's Tanongsak Saensomboonsuk and set up a quarterfinal against India's Kidambi Srikanth.
The World No. 1 from South Korea was on court for over an hour before going through 21-14, 17-21, 21-13.
"It was a very tough game," he admitted. "But I am already looking forward to tomorrow's match. The win has helped make me more confident after my recent injury because my season so far has been about recovery."
China's Lin Dan is doing it the hard way in his bid for a sixth men's singles victory.
For the second day in succession, the 33-year-old had to come from behind and spent a gruelling 81 minutes on court before overcoming England's Rajiv Ouseph 14-21, 21-17, 21-16.
"It was tough," admitted Dan, who won the last of his five titles in 2013.
"He started so quickly and it took a while for me to get a hold of the situation.
"I also started slowly yesterday, but I am just doing my best. I think I am improving in each round."
Viktor Axelsen, the top European hope, cruised into the quarter-finals with a 21-13, 21-18 victory over Hong Kong's Ng Ka Long.
Meanwhile, defending champion Chen Long and Carolina Marin, who are also the 2016 Olympic gold medallists, both cruised through to the quarterfinals.
China's Chen beat India's Ajay Jayaram 21-11 21-10 and Marin ousted the final unseeded player in the women's event, Mia Blichfeldt of Denmark 21-7 21-11.