The upset could help Lee Chong Wei, the top-seeded three times 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 number 21 from Taiwan, en route to the quarter-finals.
Lee defended economically, controlled most of the rallies at a pace at which he felt comfortable, and mixed up the patterns cleverly as he squeezed through 21-18, 21-18.
“I just tried to forget my injury,” the world number one said, when asked what he had been trying to do.
One hour after Chen’s defeat, the other Olympic singles champion almost followed him to the exit.
Carolina Marin survives scare
Carolina Marin was within two points of defeat at 17-19 in the second game against He Bingjiao, a 19-year-old left-hander who looks like China’s next great women’s singles hope, before surviving 15-21, 21-19, 21-10