Birmingham: World No.12 Tanongsak Saensomboonsuk dumped China's Olympic champion Chen Long out of the All England Open on Thursday.
The Thai won 21-16, 21-19 for the biggest victory of his career and booked a quarter-final berth in the men's singles.
Seven months and an absence of competition made Chen vulnerable and so did Tanongsak's sensible strategy of not too frequently attacking an opponent who created brick-wall mid-court defence and turned it into damaging counter-attack.
Chen fought hard to close a five-point deficit to 19-19 in the second game, and might well have improved had the match gone to a decider.
Instead on the next point he hurtled a smashed wide, and then lifted the shuttle a little too short to defend against Tanongsak's attack on match point.
Last year, Tanongsak won his maiden Superseries title at the Denmark Open in October and the success has boosted his confidence.
"I played fast and offensive. It was a good match for me and he made more errors than me," he said after beating Chen.
"I'm more confident after winning the Denmark Open."
Chen said: "Every player wants to win this title and I did too. My ambition is to get back my form."
The upset could help Lee Chong Wei, the top-seeded three-time former champion from Malaysia, who survived for the second day with an ailing knee and who might now meet Tanongsak in the semis.
Lee again proved himself once again a master of adaptability and economy while enduring the discomfort of his knee injury and overcoming Wang Tzu Wei, a young and ambitious world No. 21 from Taiwan, en route to the quarter-finals.
World No.8 Ratchanok Intanon, the 2013 world champion from Thailand, met her target of reaching the quarter-finals for the third time in a row on Thursday when she beat Hsu Ya Ching of 21-14, 21-15.
The fifth-seeded Thai, a finalist in 2013, was scheduled to face Olympic champion Carolina Marin in the last eight late last night.
Marin was within two points of defeat at 17-19 in the second game against He Bingjiao, a 19-year-old who looks like China's next great women's singles hope, before surviving 15-21, 21-19, 21-10.
Ratchanok and Tanongsak were the only two Thais left at the 2017 All England event.