The 25-year-old moved ahead of England's Joe Root (886 points) and South Africa's AB de Villiers (881) to sit alone at the top of the heap on 889 points.
Turner is the only other New Zealand player to top the ICC test batting rankings and said, when approached for comment, Williamson ticked nearly every box.
"He makes good decisions at the crease in terms of shot selection which is always pretty helpful,'' Turner said.
"He does not seem to be easily rattled. He goes about his business and seems to cancel out what I call the negative outside influences which can be opposition sledging or whatever.
"And, although it will still be improving with time, he has his own game pretty well sorted.''
Williamson's ability to play the ball so late and the urgency at which he approaches an innings were particularly impressive, Turner said.
But asked where he thought Williamson could improve, Turner felt the next challenge would be against spin bowling.
"What I'm really interested in seeing is if he gets against a couple of really good spinners on a turning track and how he deals with that.
"Occasionally, I've seen him sweep off the stumps. I've always believed that that is too high of a risk.
"If you can get down to the pitch of the ball, you should do it and he is looking to do that. Personally, I believe you should look to play forward as much as you can and only go back when you are forced back.
"He seems to be moving that way but I have not seen enough of him against spin to know whether that is the case with him yet.''
