Thomas, the former postman from Pencoed, will become the first Welshman to win 100 Test caps and he joins an exclusive club of just eight international players to have achieved the feat.
The 100 club is headed by Australia scrum-half George Gregan, who has a world record 137 caps, and includes Fabien Pelous, Jason Leonard, Phillippe Sella, Stephen Larkham, David Campese and Alessandro Troncon.
Less than two weeks ago, Thomas was left fearing he could be stuck on 99 forever after suffering rib cartilage damage in Wales' defeat to Australia that threatened to rule him out of the tournament.
But Thomas has made a remarkable recovery and replaces Kevin Morgan at full-back for Wales' must-win clash with Fiji, where a quarter-final tie against South Africa is on the line.
"I feel that a win on Saturday, with the occasion it is then for me, would be the proudest day of my life and I want it to a proud occasion not just for me but for the boys and the management team around me as well," said Thomas.
"It's a huge achievement that I'm really proud to get.
"I didn't realise that one cap was a possibility. I didn't realise that 10 caps was a possibility.
"There's been times that I played games that I didn't really deserve.
"For me it just kept coming and coming, getting closer and closer and the reason why I feel I got it is because I never searched for it and never wanted it as a personal goal.
"It's just something that has arrived.
"On days when you achieve such great things as I feel this is, you must remain true to yourself.
"In the build-up to Saturday it will be exciting for me and after the game I will probably be more excited and still thinking about the achievement.
"During the game it's never about me and never will be about me, it will always be about the team."