TOKYO, September 26, 2013 (AFP) - Katayama got off to a flying start when he birdied the opening three holes before adding five other birdies at the challenging Ibaraki Country Club.
Junhasavasdikul carded three birdies and one eagle in his first six holes before adding two more birdies against one bogey for a 65.
Japan's Masahiro Kawamura, Song Young-han of South Korea and Rory Hie of Indonesia followed with a 69 at the 150 million yen ($1.5 million) event.
Katayama has enjoyed much success earlier in his career which includes winning 26 titles on the Japan Golf Tour but not since 2008 due to injuries.
"I haven't won in a while but if I do win then it will be like a new me," he said.
"Now I'm focused on what's in front of me and hopefully that feeling will last. I do not want to think about the future," he said.
Top scores:
63 - Shingo Katayama (JPN)
65 - Pariya Junhasavasdikul (THA)
69 - Masahiro Kawamura (JPN), Rory Hie (INA), Song Young-han (KOR).
70 - Kim Sung-yoon (KOR), Prayad Marksaeng (THA), Koumei Oda (JPN), Yoshinori Fujimoto (JPN), Michio Matsumura (JPN), Koki Furuta (JPN, amateur), Lee Kyoung-hoon (KOR), Taichi Teshima (JPN).