The Thai standout was looking forward to a break after finishing 72 holes on 12-over 292 to complete a hectic five-week streak of events.
"I played so badly," Thongchai said. "I'm a bit tired as this is my fifth tournament in a row. I would not normally play five tournaments in a row. The Open wasn't on my schedule but I got in.
"Also, my neck is still not good. It feels very tight. Maybe it's because I'm tired. I've seen the physiotherapist. It's a muscle problem. It's nothing serious. I cannot turn the neck fully now.
"I'll take a week's rest. I think the rest will help me to recover."
Thongchai, who won last month's Wales Open for his first European title, will stay in England for several days before meeting his coach in Munich, then going to America for the World Golf Championships Bridgestone Invitational.
The triple Asian Tour number one's victory in Wales has given him confidence that he can win anywhere in the world now.
"After Wales, I know I have a chance to win anywhere," he said.
"My confidence is back but this wasn’t my week. I lost a bit of my golf game. I couldn’t control the ball, a lot of bogeys. A bit unlucky too as a few drives bounced into the rough."