The Chinese world number five and reigning Olympic champion lost in two games, 23-21, 21-5 to Thai player Kantaphon Wangcharoen in Bukit Jalil.

Chen, who won the World Tour event in 2015, struggled throughout the first game before collapsing dramatically in the second, winning just five points at the Axiata Arena. 

Nineteen-year-old Wangcharoenm called it the biggest win of his career.

"Chen Long is one of the world's best players and to beat him is a big achievement," he said.

"I think I focused on each point and played to my best ability."

Chen's Chinese team-mate and the defending champion Lin Dan had no such troubles in his match. 

The two-time Olympic gold medallist eased past Hong Kong's Wong Wing Ki, 21-17, 21-8, but afterwards said the early matches are "not easy".

Lin will now face All England winner and fellow Chinese player Shi Yuqi in the second round, who Lin said has "improved a lot".

Top seed and reigning world champion Viktor Axelsen of Denmark needed three games to beat Thailand's
Khosit Phetpradab, 21-17, 18-21, 21-13.

In the women's singles the world champion Nozomi Okuhara from Japan struggled in her first match against another Chinese athlete Chen Tufei, although Okuhara did manage to win the big points and eventually took the win 15-21, 21-15, 21-15.

Okuhara, who is currently ranked eighth in the world, has battled injury problems since winning the World Championships last August in Glasgow and said she still does not feel at her best.

"I don't think I will be able to win the World Championships this year, although I want to," she said.

"I don't think I'm at the level required to be the champion. 

"All the top 10 or top 15 players are very strong and anyone can win."

She will now face Thailand’s Pornpawee Chochuwong in the second round, who comfortably beat Denmark’s Line Kjaersfeldt 21-16, 21-10 today. 

Other winners on the day included second seed Akane Yamaguchi - 17-21, 21-12, 21-13 over Indonesia's Dinar Dyah Ayustine and India’s Saina Nehwal, China’s He Bingjiao and Ratchanok Intanon from Thailand.

In the mixed doubles, Denmark's new pairing of Joachim Fischer Nielsen and Alexandra Boje beat Russia’s Evgenij Dremin and Evgenia Dimova in a much closer match 21-19, 18-21, 21-10.

They will now face the Denmark Open champions Tang Chun Man and Tse Ying Suet in round two, in a match which Fischer expects will be much tougher. 

Elsewhere in the draw, England's couple of Chris Adcock and Gabrielle Adcock won a tough match against Indonesia's Praveen Jordan and Melati Daeva Oktavianti.

The Indonesians were leading the third game at first, but the Adcocks eventually caught and surpassed them to take it 21-18.

Their reward for winning is a match-up against fifth seeds Zhang Nan and Li Yinhui from China. 

The home crowd had little to cheer today. 

Their top three pairs in the mixed doubles all crashed out, though some consolation did come in the men's doubles, where Ong Yew Sin and Teo Ee Yi unexpectedly beat the second seeds of Mathias Boe and Carsten Mogensen from Denmark.

Play continues tomorrow with more first round matches taking place. 

Chinese Taipei's number one seed Tai Tzu-ying will start the day in the women's singles with a match up against Malaysia's own Soniia Cheah.

The competition will end with the finals on July 1.