Sonny Bill Williams has rarely been described as nervous across a magnetic sports career but that’s his state of mind ahead of a late bid for selection to his third World Cup.

All Blacks star Williams is set to be named at inside centre for Saturday’s Rugby Championship Test against South Africa in a rare recent appearance at any level.

Assistant coach Ian Foster said the 33-year-old impressed in a full training session on Tuesday but his hamstring will be assessed again later in the week.

The same injury ruled him out of the 20-16 win over Argentina, continuing a miserable year in which the 33-year-old started just five Super Rugby games.

Having started only 13 games for the Blues across the last four seasons, and just five Tests last year, Williams has made a habit of winning All Blacks selection off the back of his potential.

However, a logjam of midfield talent makes his World Cup berth uncertain and Foster said the former NRL star was feeling the heat in the leadup to his second full game in 20 weeks.

“He’s a bit nervous. He wants to know what’s happening and he’s like every other player now,” Foster said.

“There’s that apprehension and excitement, but at the end of the day we’ve got to get him focused on learning and doing what he needs to do then we’ll make the selections that we see as right.”

Ngani Laumape impressed against the Pumas while accomplished Crusaders back Ryan Crotty is another compelling World Cup candidate at inside centre although he is still weeks away from returning from a fractured thumb.

Foster said it was a challenging selection process as they don’t want to field a player who isn’t 100 per cent but they are desperate to see the likes of Williams in action before a squad of 39 gets trimmed to 34 for two Bledisloe Cup Tests and then to 31 for the World Cup.

“It’s a bit of a tightrope you walk. We just need to make sure when we put a player out there it’s the right thing for them.”