In his best, Williams All Blacks is at great speed in the midfield, but his form has not been outstanding this year and younger rivals for a place in the centers drive him close.
"Sonny has had the smallest rugby and is probably still finding his best form," assistant coach Ian Foster told reporters in London.
"I think it's a good thing to say. Our midfield is now jumping out of its skin. In the last two weeks, we've probably seen the energy levels and working rates in that group as good as the year round.
"The others when they went out, they've done a really good job."
Involved by many injuries, twice World Cup winner Williams played only six games for Auckland Blues and has appeared in four tests so far this year.
The 33-year-old spent 80 minutes in just one of these tests and replaced during the other three, including one where he spent 10 minutes in sinbin.
Sometimes he has reminded the rugby world what he can do with his slash that goes into space and the ability to release his hands to the ceiling and punish the defense, but those moments have been rare.
Coach Steve Hansen has said that Williams is the kind of player who needs a game contest to get the best and over time goes out to test new combinations before next year's World Cup, he is expected to get a start against England.
His credible center partner, Jack Goodhue, has obviously been the revelation of New Zealand's season and despite having played only five tests, it is increasingly likely to be the starting point at the next year's global showpiece.
Ngani Laumape also raised his hand with an outstanding screen, which included a hat trick of trials, for a second-string All Blacks side in the 69-31 spell in Japan last weekend.
Hansen, who is well aware of the strength of Williams mating with Ryan Crotty, has not been able to really see his partnership with Goodhue and the duo can be a major challenge for England's midfield in Twickenham.
Goodhue, who also crosses the line and is a calm defender, missed All Black's 37-20 victory over Australia in Yokohama last month with a disease, but Foster said he was now back to full training.
"There was some fatigue at the end of last week," said Foster. "He has had a couple of really good training days and looks like he is 100 percent." (Reporting by Greg Stutchbury in Wellington; Editing Nick Mulvenney)