Alongside his partner John Collins the pair clocked a time of 6:22.95 to finish comfortably in second place behind France.
The Preston ace, who was denied a chance of winning a medal at the Rio Olympics in 2016 after falling ill after touching down on Brazilian soil.
He has admitted to wiping away a few tears after finally getting the chance to compete at an Olympics.
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“There were definitely tears behind the sunglasses,” Thomas admitted. “Very emotional to be honest.”
The final takes place on Wednesday.
Chorley swimming ace Anna Hopkin was part of the Great Britain 4x100 freestyle relay team which just missed out on medal.
The 25-year-old along with Lucy Hope, Abbie Wood and Freya Anderson came fifth in new British record time of 3:33.96.
Australia won the gold medal in a world record time of 3:29.69, which was more than three seconds ahead of second placed Canada and the USA, who took bronze.
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