SINGAPORE - The two defending champions of the OUE Singapore Open have been unceremoniously dumped out on their first day of competition.

Both 2016 winners, Sony Dwi Kuncoro of Indonesia and Thailand's Ratchanok Intanon failed to progress beyond the first round on Wednesday after falling to less heralded opponents.

Men's singles winner Sony was taken to three sets before losing 21-19, 15-21, 18-21 to South Korean Lee Dong Keun in 76 minutes, the competition's longest game so far.

World No. 7 Ratchanok fell 8-21, 18-21 in straight sets to Japan's Sayaka Sato.

The Thai, who has been hampered by an ankle injury suffered at last month's India Open, credited her world No. 19 opponent for the upset.

The former world champion said: "She had a good game, good speed, and good defence. In the second game, I (tried to) come back, but was not confident in myself, and the (points) gap was too far. I also had to take care of myself."

It proved to be a day of upsets, as world No. 2 and Olympic bronze medallist Viktor Axelsen was also booted out of the competition, losing 15-21, 15-21 to Hong Kong's Vincent Wong in his opening match.

The Dane admitted that three weeks spent competing in Asia has taken a toll on him, and he struggled to deal with the draught and committed many errors.

In another big clash of the day, India's Olympic silver medallist and world No. 2 P.V. Sindhu overcame a sluggish start, coming from behind to edge out Japan's Nozomu Okuhara 10-21, 21-15, 22-20. The loss left the world No. 10 in tears.

Singapore's challenge in the women's singles also ended as 18-year-old Yeo Jia Min was defeated by South Korean Kim Hyo Min 21-15, 21-17.