China's Wang, the gold medallist in both the women's singles and team events at the Incheon 2014 Asian Games, suffered the recurrence of a back injury, causing her to retire during her match against Chinese Taipei's Tai Tzu Ying, which was at 21-12, 11-8.

"I haven't been well - apart from the back injury, I fell sick during the China Open and lost a lot of weight, so I have been weak," Wang, the 2011 world champion, said at the Hamdan Sports Complex.

"I'm not as young as I used to be.

"Now it's harder to recover as I'm older.

"I'm okay with it because I cannot change some things.

"Other teams have been picking up.

"They have younger players who can learn things very fast, that's the advantage they have.

"When we were young we picked up a lot from our rivals.

"Failure is not always a bad thing and learning from failure is crucial.

"So we will try to get back.

"I want to recover from injuries as soon as possible so I can get ready for the [Rio 2016] Olympics."

Mathias Boe and Carsten Mogensen defeated Japan's Hiroyuki Endo and Kenichi Hayakawa 16-21, 21-19, 21-16 ©BWF


South Korean Son, meanwhile, retired when he was 21-9, 11-3 down to Japan's Kenichi Tago due to a hamstring problem.

"This was an old hamstring problem that flared up recently," Son said.

"I've had a good year.

"The Asian Games team gold medal win at home was most memorable, but it has meant additional pressure."

Elsewhere, Japan's Akane Yamaguchi continued her good form and secured her second straight victory with a 21-15, 16-21, 21-12 win over Thailand's Ratchanok Intanon, as did Indian Kidambi Srikanth, who defeated Indonesia's Tommy Sugiarto, 21-18, 21-13.

Japan's Kento Momota bounced back from his opening loss to secure a 20-22, 21-18, 21-12 win over Jan Ø Jørgensen of Denmark.

"Jørgensen has great stamina and sharp strokes, so I was trying to avoid mistakes," Momota said.

"I had to be patient.

"I was trying not to think of the first game loss."

The day also saw the upset of women's singles top seed Wang Shixian of China to South Korea's Bae Yeon Ju 21-17, 21-17.

In the men's doubles there was joy for Denmark's Mathias Boe and Carsten Mogensen, who defeated Japan's Hiroyuki Endo and Kenichi Hayakawa 16-21, 21-19, 21-16, while their compatriots Joachim Fischer Nielsen and Christinna Pedersen also tasted victory by overcoming Indonesia's Tontowi Ahmad and Liliyana Natsir 21-15, 22-20 in the mixed doubles.

Hans Kristian Vittinghus, however, could not continue the Danish winning streak as he went down 13-21, 21-10, 21-9 to China's men's singles top seed Chen Long.

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