The Springboks' year to date has been dominated by injuries to key players, creating much uncertainty for coach Heyneke Meyer in selecting his final 31-man squad.
But having included the likes of captain Jean de Villiers, loose-forward Duane Vermeulen and scrumhalf Fourie du Preez, who have just a single test appearance in 2015 between them, the gamble appears to be paying off.
"At this stage everyone will be fit and ready for that first game," Roberts told reporters on Monday.
"Obviously we're taking it day-by-day, but that is our plan, to have 31 players fit and ready for the coach to select the 23 he wants for his first game."
The experienced Du Preez remains the biggest concern, but Roberts says he is getting better with each session.
"Fourie has done really well, obviously he hasn't played a lot of rugby but for him we are trying to get him playing more and more simulated game time in training.
"He did the full training today including the contact session and took some nice hits. We're building his confidence and he came through the training nicely.
"I'm happy with his progress, we're still two weeks away and he is doing really well."
South Africa's last test was the 26-12 victory over Argentina in Buenos Aires on Aug. 15, their only win of 2015 to date. It means they will not have played for over a month by the time they tackle Japan in Brighton, but Roberts does not feel this will have an effect on the match fitness of the players.
"It's always a catch 22 because there is a difference between training fit and match fit," he said.
"But when guys have 50 caps and are experienced they generally get that match fitness and running lines a lot quicker. I'm generally comfortable with where we are.
"If we were playing in games this last weekend and we were picking up injuries, then I'd be worried to lose players in warm-up games which are not necessary."
Wing Bryan Habana and centre Jean de Villiers sat out Monday's training session with flu, while lock Eben Etzebeth has a calf strain and a stomach illness.
South Africa are in Pool B with Japan, Samoa, Scotland and the United States.
(Reporting by Nick Said; editing by Martyn Herman)
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