Mollie O'Callaghan and Shayna Jack fire warning shot with sensational 1-2 heat finish

Australia's freestyle sprint stars Mollie O'Callaghan and Shayna Jack have sent a warning to the field, comfortably taking out the top two spots in their women's 100m freestyle heat.

Fresh off her sensational women's 200m freestyle gold medal swim – which saw O'Callaghan take out the gold with fellow Dolphin Ariarne Titmus securing silver – O'Callaghan finished with a time of 53.27.

Jack, meanwhile, finished milliseconds later, clocking in at 53.40.

Watch the video above.

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The sprint stars cruised through to the semi-finals (Wednesday morning AEST), with the final then scheduled for Thursday.

"That was good from Shayna Jack," Nine commentator Ian Thorpe said of the athlete.

"She looked like she was struggling to make up the ground on the second lap. But as the wall approached, she increased the tempo and she's finished in second position behind the gold medallist from last evening."

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"Very comfortable swim from both our Aussie women," fellow commentator Giaan Rooney said.

"We are no problems with the semi-final tonight."

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The current world record holder for the event is Sweden's Sarah Sjöström, with a time of 51.71.

Sjöström won the second heat on day four in Paris, clocking 52.99 – qualifying for the semi final 0.28 faster than O'Callaghan.