James said: "I think we have completed the toughest pre-season fitness training I have ever been involved in during my career so far. The fitness levels are there for us now and all the hard work has brought us together as a group."
He added that Wales were aware of the threat posed by the powerful South African forwards and said the squad had also worked on avoiding giving away penalties.
James explained: "We can't afford to give silly penalties away and that is something we have been working on in training. If we can match them up front it's going to help us give our backs the sort of ball which will allow them to play better rugby."
Wales defence coach Shaun Edwards predicted the South Africa match would involve a huge commitment from the loose forwards from both teams.
He said: "It's going to be a big tussle for our loose forwards but that is only what you would expect in a game like this. The South Africans are very good from set piece so we have had to work very hard on this. We know they are incredibly big and powerful men
"(Heinrich) Brussow is a genuine world class 7 and a real snapper when the ball is on the floor. He's still turning ball over even under the new interpretationas of the law.
"He's a real snaffler on the floor. In 2009 (during the British and Irish Lions series) the way the game was refereed then you didn't have to release when they hit the floor so that was to his advantage.
"But to his credit he is still turning ball over under the new interpretations. He has adapted his style and technique. It's going to be a big challenge for our loose forwards but that's what you would expect playing against the world champions."
Wales play South Africa in their opening pool clash at the Wellington Regional Stadium on Sunday.