World No 1 Li Xuerui suffered a shock defeat in the second round of the All England Open on Thursday, losing for the first time to Chinese compatriot Sun Yu 21-13 21-13.
Unseeded Sun had never taken a game off her compatriot in four previous matches.
The departure of Li, 2012 champion, left the top half of the draw open to 2009 champion Wang Yihan, Saina Nehwal of India and 2013 world champion Ratchanok Intanon.
Li admitted she had been struggling with match fitness and said: "I still haven't recovered from my foot injury.
"I have only been in full training for two weeks, but I came to this tournament because I love it, and I never expected to win the title. I am disappointed, but it's not a major setback, as I came here to enjoy it."
England's top men's singles player, Rajiv Ouseph, maintained his superb form to reach the last eight for the first time with victory over India's HS Prannoy.
Ouseph, the world number 22, followed up his stunning defeat of third seed Son Wan-ho by coming from behind to see off Prannoy 17-21 21-19 21-18.
The English national champion will now face Japan's Sho Sasaki for a place in the semi-finals.
Ouseph, who has overcome a leg injury to compete in Birmingham, said: "For me to get through to the quarter-finals, it hasn't quite sunk in. I am playing as well as I have ever done and long may that continue."
Defending champion Chen Long looked in imperious mood, easing past Taiwan's Jen Hao Hsu 21-7 21-9.
Watch the All-England Open semi-final live on SportNews 3 on Saturday March 7 from 12pm. Then watch the final from 12pm on Sunday March 8, live on SportNews 3.
