Even when scans proved the injury was not as bad as first expected, there was only thought to be an outside chance Thomas would be fit to face the Fijians.
But he has been in full contact training this week and replaces Kevin Morgan at full-back to bring up a century of Test caps in a must-win encounter for Wales.
At stake is a place in the World Cup quarter-finals and head coach Gareth Jenkins said: "No side would want to go into a game of this magnitude without its captain, it's as simple as that.
"Gareth's importance to this team cannot be understated, he's reached an historic landmark but no-one will be more focused than him on match day.
"His stature and composure on the rugby field will be invaluable to us in a game that could offer us such high rewards.
"We are within touching distance of a place in the Rugby World Cup quarter-finals, we need to respect what Fiji offer and maintain our focus and our professionalism throughout and there is no reason why we cannot come away with a win."