Moriarty knows all about the threat of Fiji from his own experience.
And that is why he has warned his team they must be firing on all cylinders when they take on the Pacific Islanders.
In 2012, Moriarty featured for Gloucester A against Fiji A at Kingsholm and helped the English outfit to a famous and thrilling 31-29 success following a second-half fightback.
"Any game at the World Cup is big. I was fortunate to play for Gloucester's A team against Fiji A when I was 18 which was a bit of an eye-opener," Moriarty told the Welsh Rugby Union's official website.
"To be on this stage now makes it even bigger and I'm excited to get out on the pitch, get a start, and show everyone what I can do.
"We know it's going to be tough. It won't be easy, but we've been working hard in the last week and we've done our analysis.
"We will have to close Fiji down in defence and in attack we want to get points on the board.
"We know they have some great individuals, but so do we and we're just going to make sure we put everything out there and do the job."
Moriarty starts at number eight against Fiji after coming off the bench in the first two tournament wins over Georgia and Australia.
It means Josh Navidi has moved from the base of the scrum to the blindside flank with James Davies starting on the openside.
"Obviously I've been disappointed not to get the starts, but nonetheless this is a World Cup and it's great to be involved, to come off the bench, and to contribute in some big games," said Moriarty.
"I've got the start now and I'm really excited for it. I've been working hard in training and it's been great competition in the back-row. We're all fit and firing."
Wales completed their captain's run training session at Oita Stadium on Tuesday on the eve of facing Fiji.
With a capacity of 40,000 and a roof, humid conditions have characterised the World Cup games at the venue so far.
"We're very aware of [the humidity] because our own stadium has a roof on it," said Wales forwards coach Robin McBryde.
"We've come across humid conditions even back at Cardiff. We've got to go out and enjoy the match as well.
"We're in a good place and we've obviously had a 10-day break. We've been able to get recharged. We're ready for the final push with two big games.
"Fiji had a strong set-piece at the last World Cup. They're big men. We're aware of that threat. We're not taking anything too lightly and that's an area we've been focusing on.
"We've created our own bubble at the World Cup. Historically we always get stronger the more time we spend with each other.
"The environment we've created allows individuals to improve their game, but collectively it makes us stronger as a group as well.
"That collective nature will get us through the next game against Fiji."