Three-time champion Lee Chong Wei was sensationally beaten by India's B Sai Praneeth in the first round of the All-England Championships on Wednesday, putting a firm dent into his preparations ahead of the Rio Olympics .
The Malaysian second seed was beaten 24-22 22-20, which ended a promising few months where he had won his last four events.
"I can't believe I lost in the first round," Lee told reporters. "I had pressure to win but I made mistakes and I'm shocked at how well he played.
"This is a real learning curve for me."
Lee, the former world number one, was playing his first All-England since 2014 following his eight-month drug suspension ended last year.
Praneeth, however, clawed his way back from 11-3 and 15-7 deficits in the first game to take control of the match and hailed the win as his best result.
"It is a big shock for me too and after the last rally I cannot believe that I won," said Praneeth, who set up a last 16 clash with Denmark's Hans-Kristian Vittinghus.
"I do not know how I turned it around. I think he was under pressure as it was a close match and that was a big advantage for me. Of course it is my greatest victory of all time."
Earlier, world champions Chen Long of China and Spain's Carolina Marin were tested as they got the defence of their singles titles underway with contrasting wins.
Chen to beat Japanese qualifier Kenta Nishimoto 21-18 21-12 while Marin held off South Korea's Bae Yeon Ju in a marathon 88-minute, error-prone encounter.
"The first game was challenging and I'm glad I stayed really calm," said Chen, aiming to win his third title in four years.
Unlike his chief rivals for the men's title – compatriot Lin Dan also eased into the second round with a 21-17 21-17 victory over South Korea's Lee Dong Keun – Chen was playing his first major match of the season.
"I played some matches in the national league but it's very different to playing international matches," added Chen.
"That's why we sometimes take a bit of time to settle into our first overseas matches," he added.
Marin, who has broken China's domination to reach world number one, made several uncharacteristic mistakes at the net.
She had to save three game points in the opener, failed to finish the contest with two match points in the second, and then came from 9-6 down in the decider to win through.
"It's been a long time since my last Superseries event and I've just recovered from a shoulder injury," Marin said after a 25-23 20-22 21-15 victory. "I have to cut out the mistakes."