Belgium, who has played less two games from Wales, will travel to Cardiff for return on 13 October.
Thomas said that Belgium did not show Wales “very respect”, which was 3–0 in front and the domestic side thought “the game was done”.
But Thomas said: “To be honest, we still believed when they went above 3–0.
“If you watch the game, every time they score, we really went down from the other end and got almost a chance.
“I think half-time are going on 3–1, he thought that the game was done. I think they rested and they underestimate us.
“The second half shown that there was only one team in the game and she was us.
“This is a learning state for us – this is a young team and we are going to places. It is a disadvantage in 10 and we are going to learn from it and are getting better.”