The 35-year-old is Wales' all-time leading try scorer but currently plies his trade in Japan for Mitsubishi Dynaboars.
Wales currently sit sixth in the world rankings and will undoubtedly head to Tokyo next year as firm favourites.
But Williams is warning against complacency ahead of the trip, especially since he was part of a Wales team in 2001 that suffered an embarrassing defeat in Suntory in their opening tour game before going on to sweep a two-Test series.
"I know a lot of Welsh players are looking forward to the tour next season," he said. "For me, Japan probably is one of the most hospitable places in the world, and certainly for a young squad coming on tour, there is no better place to come.
"I remember enjoying the experience when I came to Japan in 2001 on tour. I certainly didn't expect what happened to us there.
"We went there thinking we would come and win every game but unfortunately our first game was against Suntory, in 35 degrees and 90 per cent humidity and we lost.
"That was a long time ago and since then the standard of rugby in Japan has doubled and the quality of players has doubled. The Top League and other leagues below it are far better than 11 years ago.
"As for the national side of Japan, I think they're progressing every year. I certainly enjoyed watching them during the World Cup. I think they showed a brand of rugby that excited the world. Under Eddie Jones, they're continuing to progress.
"They have some exciting players. What I do like most about Japan national side and Japanese sides is that they do want to play rugby and score tries."