On Wednesday night, Lee Chong Wei, the world number two from Malaysia, had also been beaten by an unseeded player, falling in the first round to B. Sai Praneeth of India.

The curiosity of Thursday’s upset was that Chen, usually such a consistent player, was uncertain and error-prone at the most important moments – from 19-18 up in the first game and from 17-17 in the second.

He was a little too inconsistent at the net and in mid-court – usually an area of great strength for him – and he was also often unable to force attacks home, perhaps because of the cool conditions.

Chen also tried to claim the result was not so surprising.

READ MORE: Malaysian ace Lee Chong Wei upset in opening round at All England Championships

“It can happen because the competition is so fierce,” he said. “It’s normal, it can happen.”

Attempts to find out how long it had been since the top two seeds were beaten so early suggested that this was not so.

Even more disorienting was the low key reaction from the winner.

“It’s no secret that our relationship is so close,” Xue said. “It is closer than with my parents – and I think that impacted on our performances.

“I think it impacted him as well as me, and I am not feeling great about winning this. Although he is the top seed I was not desperately wanting to win.”

Chen’s position as world number one is not immediately under threat after a superb 2015 in which he won seven major titles, but the surprise result does appear to open a path for another compatriot, Lin Dan, to regain the All-England title and win it for a sixth time.

Lin reacted to danger quite differently, switching to the overdrive gear to accelerate into the quarter-finals.

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The outcome was impressive as the two-time Olympic champion utterly transformed his match against Sho Sasaki, the world number 20 from Japan, finishing a calm 20-22, 21-6, 21-8 winner.

He played at a higher speed, he sliced and disguised smashes into difficult areas, and he gave Sasaki far fewer opportunities to win points at the net, where the Japanese player had done well.

Asked how the Lin Dan of today compared with the Lin Dan who won the gold medals in Beijing and London, he replied: “There are more and more challenges. It’s important to keep on training to get the best out of myself.”

Women’s top seed Carolina Marin suggested she had recovered from her narrow first round escape against Bae Yeon Ju of Korea by outplaying Maria Kusumastute, the world number 20 from Indonesia by 21-17, 21-7.

“Yesterday I just wanted to get confident. Today was a very different match. Today I was happy with my performance and how I could cope,” the Spaniard said.

Marin’s success will bring what may be another emotionally uncertain match between friends, for she next plays Ratchanok Intanon, the former world champion from Thailand.

“Carolina is a close friend – we used to train together,” Intanon said after trouncing Akane Yamaguchi, the teenaged world number 11 from Japan, by 21-9, 21-13.

“It will be a difficult match but if I just focus I think everything will be okay.”