Liam Williams' injury-enforced absence from the Rugby World Cup semi-final against South Africa is "a big loss" for Wales, but Warren Gatland does not think his team is any weaker.

An ankle injury sustained in an accidental collision during training ruled Williams out for the remainder of the tournament in Japan, with Leigh Halfpenny taking his place in the XV for Sunday's match in Yokohama.

However, Gatland stated Halfpenny had been close to earning a starting spot regardless and believes his experience will be an important boost for Wales.

"[Williams] is undoubtedly a big loss from an attacking perspective and what he has achieved in the game in the last year or so," said Gatland.

"But bringing in the experience of someone like Leigh Halfpenny gives us a different element.

"He is defensively probably the best full-back in the world in terms of his aerial game and coverage defensively.

"We had a long debate about whether we started Leigh in the first place and potentially move Liam to the wing.

"There was a long discussion about that so Leigh was probably unlucky he was not in the team in the first place.

"We are disappointed for a world-class player like Liam but we are happy bringing in someone with the experience of Leigh.

"It is a change but we don't think that we are weakening the side in any way with the changes we have made."

Jonathan Davies will start at centre after overcoming a knee injury and Gatland is pleased to have him available for the first time since the pool stage win over Fiji.

"We hope he is fit. He has come through training this week," said Gatland. "He was very close last week and he made a decision last week to rule himself out for the betterment of the team.

"Hopefully we can get through the rain today with the training and it won't affect us too much.

"We will have a shorter session this afternoon and we are excited where we are at the moment."

South Africa were dealt an injury blow of their own with livewire wing Cheslin Kolbe missing out after aggravating an ankle injury in the quarter-final success over Tokyo.

"Obviously he is a big loss to them with his ability to score tries and his footwork," Gatland said of Kolbe, for whom Sbu Nkosi will deputise.

"We were probably going to target him aerially anyway. You talk about their kicking game, I think our kicking game has to be good as well. They definitely have a bit of pace in the back three.

"At this stage of the tournament, you are always going to lose a couple of quality players, and in them losing Kolbe and us losing Liam Williams, it's probably one each in terms of that."