Denmark's third seed Anders Antonsen was knocked out of the men's competition by home player Lee Zii Jia, as singles action began at the Badminton World Federation (BWF) Malaysia Masters.
Last year's World Championship runner-up was a big first-round casualty at the Axiata Arena in Kuala Lumpur, which is hosting the first event of the new BWF World Tour season.
Jia came from behind to win in front of his home crowd, 14-21, 21-15, 21-8.
"I'm not sure what happened," Antonsen said.
"I think he suddenly started playing better in the second game.
"Maybe I didn't have the rhythm as this is the first match of the year.
"I will have to analyse this and hopefully do better in Indonesia next week."
Back-to-back world champion and world number one Kento Momota had no such concerns, as Japan's top seed saw off India's Parupalli Kashyap 21-17, 21-16.
China's reigning Olympic champion Chen Long, the fourth seed, knocked out last year's Malaysia Masters winner Son Wan-ho in a repeat of the 2019 final.
Chen turned the tables with a 21-16, 21-14 success over the unseeded South Korean.
Chinese Taipei's second seed Chou Tien-chen was another to go through, defeating India's Srikanth Kidambi 21-17, 21-5.
Denmark's 2017 world champion Viktor Axelsen, the fifth seed, was similarly comfortable in a 21-13, 21-8 win over Indonesia's Tommy Sugiarto.
In the women's draw, Chinese Taipei's top seed Tai Tzu-ying saw off Chinese qualifier Zhang Yiman 21-14, 21-10.
China's second seed Chen Yufei needed three games to beat Denmark's Line Kjærsfeldt, 21-12, 20-22, 21-12.
A big first-round clash between Spain's reigning Olympic champion Carolina Marin and Japan's fourth seed Akane Yamaguchi went the way of the Rio 2016 winner, 21-4, 24-22.
Marin is yet to climb back into the seeded positions after missing eight months of last year with a knee injury, but demonstrated her return to the top of the sport by winning the China Open in September.
Thailand's back-to-back defending champion Ratchanok Intanon, seeded fifth, came through a battle against Indonesia's Gregoria Mariska Tunjung, 22-24, 21-19, 21-15.
Reigning world champion PV Sindhu, the sixth seed, beat Russia's Evgeniya Kosetskaya 21-15, 21-13.
The World Tour Super 500 tournament is running until the finals on Sunday (January 12).