Yokohama - An emotional Sonny Bill Williams has revealed the huge relief he felt taking the field for New Zealand against South Africa in their Rugby World Cup opener, having overcome multiple injuries to once again appear on rugby's biggest stage.

The 34-year-old centre, renowned for his audacious off-loads, admitted there were times in the past four years when he questioned whether his body would hold up long enough to make the International Stadium in Yokohama on Saturday.

"I won't lie, with the amount of injuries I've had in the last few years, of course I've questioned myself," Williams said after the All Blacks launched their bid for a third consecutive World Cup title with a 23-13 victory over their arch-rivals.

"I never questioned my ability, I questioned the body at times. I was grateful that it has come through."

Since helping New Zealand to the 2015 World Cup title, the injury-prone Williams has played a combined total of only 40 club, Super Rugby and Test matches and under the spotlight against South Africa there were signs of rustiness.

In a position Williams once commanded as his own, it was Ryan Crotty who was the preferred starting inside centre, and when Williams came on in the second half he fluffed a few lines and had a kick charged down, which did not help the All Blacks cause.

Before the match the rumour mill was swirling with gossip that the 191 centimetre (6ft 3in), 110 kilo (242lb) former heavyweight boxer and rugby league star was injured again despite the team management saying he was training well.

Coach Steve Hansen said he sees a role for the solid midfielder as an impact player off the bench, a role he also filled when New Zealand beat Australia in the 2015 World Cup final.

"As we all know, he has a different skill set than most. Being able to bring him into the game at different points is worthwhile for us," Hansen said.