Coach Gareth Jenkins said some of his players were caught up in the occasion as Wales had to come from behind to beat Canada in the World Cup on Sunday.
Gareth Jenkins endured a nervous first half against Canada
"It wasn't the greatest of starts - there were certain players affected by the occasion," he said.
"We were allowing Canada to play the way they wanted and weren't influencing the management of the game properly."
Jenkins said the introduction of Gareth Thomas and Stephen Jones early in the second half helped change the game.
"Our first half was poor but what was important was that we kept our composure," Jenkins added.
"We had a strong bench and I think the bench players impacted the game and brought a bit of confidence and belief on to the field.
"In the end we saw 30 minutes of rugby from ourselves.
"It's great to win, it's the start we wanted. I said if we had the performance I'd be happy, but a win was the most important thing."
But Jenkins has warned they will have to play better when they face Pool B favourites Australia in Cardiff on Saturday.
"We need to manage ourselves better for 80 minutes," he said. "We can't expect to win higher quality games by playing 15 to 20 minutes of rugby.
"Australia is going to be a different level of rugby and I don't think we are going to get away with performing like the way we did today if we have got an aspiration of beating them, that's for sure."
Double-try scorer Shane Williams said Wales had to learn from their slow start.
"There were some nervous times out there," he said. "We knew the mentality Canada had but they doubled it. We made far too many mistakes in the first half.
"We gave away too many penalties in silly places where we didn't need to. We lost territory and it meant they got momentum and they scored tries.
"We started to compose ourselves in the second half, go through the patterns and the gaps were eventually there. That's what we've got to do against teams like Canada.
"It was maybe the kick up the backside we needed."
Wales squad captain Gareth Thomas said he was content with the win.
"We set ourselves the objective of moving smoothly into the Australia game and the second half provided that," he said.
"I don't think we played to our full potential in the first half - but give credit to Canada, they probably deserved to lead at half-time."
And Dwayne Peel, who captained the side for the first time in Thomas' absence, added: "We knew if we stayed calm and played to our game plan, we would come away victors and in fairness to the guys, we did that.
"We spilled a few balls that we shouldn't have and that gave Canada a bit of an incentive.
"We were not as clinical in the first half as we'd have liked to have been but we spoke about finishing off half-chances at half-time and we did."