Williams, who starts the game on the bench, said despite his age he will be surrounded by experience in the Wales backline.
Williams said: "I have played alongside Rhys a lot of times and he has got all the qualities. On his day he can be one of the best number tens in Wales.
"Our 10 may be younger than his South African opponent but we have got a lot of experience in our backline."
Williams spoke of his own delight at being involved in Warren Gatland's squad and his determination to surprise a few people at his first Rugby World Cup.
He said: "I was surprised to be selected but now I am just happy to be involved. The boys are confident we can do well and the preparations have been truly excellent. South Africa are a physically big team but we have the players to deal with that."
Williams has made a huge impression since embarking on his senior rugby career with the Scarlets and Wales.
"I've improved my game so much by being involved and just learning from the other players around me," he said.
Williams praised Warren Gatland and his coaching team and said he has now developed a hunger to get more Wales honours.
He said: "Once you have put that jersey on once you just want to put it on again and again. You don't want to be left out - you just want to keep playing.
"It hasn't really sunk in how big this competition is yet so for now I am just trying to take it in my stride. Nobody knows too much about me at the moment but I am determined to make my mark."