The third-ranked Chen lost in the first round in the same Wembley Arena at last year's world championships. That was a surprise, and he was prepared for Boonsak in their group elimination match, though it took Chen almost an hour to put out last month's Singapore Open champion.
Former No. 1 Lee Hyun-il of South Korea, who was fourth at the Beijing Games, beat Rodrigo Pacheco Carillo of Peru in two games. Sixth-seeded Sho Sasaki of Japan also won in two games, as did ninth-seeded Simon Santoso of Indonesia, on his 27th birthday. Santoso planned to celebrate only with a dinner with teammates.
Other winners included Tien Minh Nguyen of Vietnam and Derek Wong of Singapore.
In women's play, third-seeded Li Xuerui of China advanced with her second group win over Carolina Marin of Spain 21-13, 21-11. Li earned two days off before the last 16 begins.
"I'll continue to practice a lot, rest a lot, and eat a lot of rice," Li said.
All the seeded players in action won, including No. 4 Saina Nehwal of India, No. 9 Ratchanok Intanon of Thailand, the three-time world junior champion, No. 12 Sayaka Sato of Japan, No. 13 Juan Gu of Singapore, and No. 15 Petya Nedelcheva of Bulgaria.
No. 14 Jie Yao of the Netherlands also won, with husband and coach Eric Pang in her corner.
Pang was ranked high enough at No. 40 to qualify for the men's singles, but Dutch officials set their criteria higher and decided not to enter him. Rather than be disheartened, Pang said he will aim for the Rio Games in 2016.
"This will be Yao's last Olympics and I want to go to Brazil," he said. "Maybe she will be my coach."