The 28-year-old admits he felt 'terrified' as he entered Stade Marcel Michelin to face Montferrand - these days known as ASM Clermont Auvergne - when he was a teenager playing for Swansea in the last season before regional rugby in Wales.
"The first game I played in Europe was as a 19-year-old with Swansea in Clermont Auvergne. It was terrifying," said Thomas.
"We went out of the changing rooms into the stadium and all I could think was 'Oh my God' - it was the most intimidating atmosphere I'd ever encountered. But it was a great l blooding into European competition.
"I played five matches that season for Swansea before the game went regional in Wales and I started my career with the Ospreys. It wasn't until our final Pool game in 2003/04 that I tasted my first victory in Europe.
"The games in France five or 10 years ago were absolutely brutal and none of the Welsh regions did very well out there. But year by year we have improved and there is far more belief when we travel these days.
"The more you play teams, the more accustomed you get to them and I think the contact we have had with the top sides in France has had a big effect on Welsh international team performances against them as well."
Thomas, like most Welsh players, harbours very real ambitions to be part of the first Welsh side to lift the biggest prize in European rugby - the Heineken Cup.
He admits the Ospreys have been close but not close enough but hopes that all changes this season.
"Everything seems to go up a notch in a Heineken Cup week and you can tell the difference at the first training session on a Monday," said Thomas.
"We've progressed as a squad at the Ospreys and when you look back we've probably missed a few chances to do better than reaching the quarter-finals.
"The quarter-final against Saracens at Vicarage Road in 2008 taught us a good lesson and showed how big a part emotion can play in sport.
"We'd beaten them by a big margin a few weeks earlier in the EDF Energy Cup and went to Watford as firm favourites. In the end, we lost 19-10.
"That proved what the Heineken Cup can do to a team - they rose to the occasion, while we were probably guilty of thinking we only had to turn up to get another good result against them."