The Celtic Warrior believes the plaudits Wales earned for their performances in the defeats against New Zealand and England will count for little at the Millennium Stadium.
"We had a lot of positives in the World Cup but I don't think you can go into a tournament like this thinking about what has been in the past," said Thomas, part of the side that out-scored England three tries to one only to lose the quarter-final.
"I think we need to get confidence not from the World Cup but from Saturday. If we can end the first game in a confident mood then we can take it on for the tournament."
Thomas believes that Wales would be better served recalling last season's RBS 6 Nations for inspiration rather than remembering events Down Under.
The Welsh never recovered from the shock 30-22 reverse to Italy and went on to lose the rest of their RBS 6 Nations games, including a demoralising defeat in Edinburgh by the same score.
"We believe we can win but the same happened last year. We were very confident going into the Italian game but we came out with a loss."
Thomas also insists he has no problems switching from his usual club role in the centre to the full-back berth he assumed with aplomb in the later stages of Wales' World Cup campaign.
The 29-year-old will form a potentially-devastating trio with wings Rhys and Shane Williams knowing he needs just one more try to equal Team RBS Expert Ieuan Evans' Welsh test record of 33.
"I'm comfortable at full-back," said Thomas, who will win his 73rd cap.
"I don't see the big deal. The last 6 Nations game we played with three full-backs. Playing on the wing or playing at full-back, there is not that much of a difference.
"I felt very comfortable going into the New Zealand game because I did not get all the pressure and all the hype that seems to be coming at the moment."
Coach Steve Hansen added: "Everybody's asking me why Gareth is going back there but he played full-back against New Zealand and England and we haven't had any test matches since.
"He has done a good job for us there before and were asking him to do the same. But he is a very capable rugby player, one of the best I have ever coached."