Singh has an injured trigger finger and Thongchai twisted his right ankle but they will play through the pain barrier at the 150 million Yen (approximately US$1.9 million) event sanctioned by the Asian Tour and Japan Golf Tour starting on Thursday.
Thongchai and Singh will make up a total of five Asian Tour Order of Merit winners this week which includes Filipino Juvic Pagunsan, Liang Wen-chong of China and Thaworn Wiratchant of Thailand, who have a combined total of eight Order of Merit crowns.
Title holder Tetsuji Hiratsuka, Gaganjeet Bhullar, who won his third Asian Tour title on Monday, Panasonic Ambassadors Ryo Ishikawa and Anirban Lahiri and current Order of Merit leader Marcus Fraser of Australia will also feature at the Higashi Hirono Golf Club.
Singh, who is a two-time Asian Tour Order of Merit winner, did not touch his golf clubs for four weeks as he injured his trigger finger in his play-off victory at the Scottish Open. Despite the lack of competitive golfing, he carded a tied 31st result in Japan last week and hopes to spring another surprise this week.
"I was pleasantly surprised at how I played last week considering I didn't touch a club for four weeks. It is always a treat coming back to Japan," said Singh, who has won four titles in Japan.
"Yes, it still hurts (trigger finger) and it is on the way to healing. Four weeks of not touching the golf clubs helped but I still have to tape it up. Hopefully it will slowly get better. I did nothing during the four weeks! I spent a lot of time with my son, wife and the family. It was good fun," smiled the 40-year-old.
Thongchai needs another month to recover from a twisted ankle sustained at the WGC-Bridgestone Invitational in August but could not pass up on the opportunity of equalling Thaworn's record of 14 Asian Tour titles with a win this week.
"Before I get my injury, I was very comfortable with my golf swing. But I couldn't do anything when I had the injury. Luckily it was on the right leg because if it was on the left leg, there would be no way I could play. This course is very hilly and I have to be careful," said Thongchai, who won his first title on European soil at the ISPS Handa Wales Open earlier this year.
Hiratsuka, a four-time Asian Tour winner, earned a dream victory in last year's edition and is not discounting himself from a successful title defence in front of his adoring fans.
"Obviously, I would like to successfully defend my title this year because I want to perform in front of the local crowd. My form is getting better. I hurt my back in June and have been struggling with the pain. But it is slightly better now after I went for a two-week massage treatment," said Hiratsuka.
His countryman Ishikawa hopes to win the perfect birthday gift as he celebrated his 21st birthday on Monday but knows he will face strong challenges from the Asian Tour stars.
"Obviously, it is important for Japanese players to win but there are so many good players on the Asian Tour. The Indians, Thais and Koreans have all improved tremendously but I'll try my best to win it," said Ishikawa, who celebrated his birthday with his family.
Fraser is in superb form with three top-five finishes on the Asian Tour. He finished second on the Asian Tour Order of Merit in 2010 and suffered a disc prolapsed in his neck in the following year which nearly forced him into an early retirement. He credited his new outtake in life for his solid results.
"I was always fairly negative sort of player but once you have an injury like that where you can't play the game then you start to realize how much you miss it and enjoy it. It was a big wake up call for me and I'm lucky be still playing the game. It was pretty scary but it is good now and hopefully it will improve," said Fraser, who has one win on the Asian Tour in 2010.