She and her coach Dean Boxall formulated a modified training program and talked about how she could try and get back to full fitness to compete at her second world swimming championships.

Given the preparation, O’Callaghan’s performance on night four was nothing short of extraordinary as Australia continued its dominance by solidifying top spot on the medal table, with six golds and three silvers.

China is in second spot (four gold, four bronze), while the USA has produced only half the winners Australia has (three gold, seven silver, seven bronze).

Sam Short broke Grant Hackett’s Australian record in the men’s 800m freestyle to pick up a silver medal before Australia’s mixed medley team came within half a second of another gold medal on another confidence-boosting evening for the Dolphins.

However, the story of the night was O’Callaghan defeating Titmus by less than a metre in an epic race that will be even more tantalising by the time the Paris Olympics begin in exactly 12 months.

When Titmus smashed the 400m freestyle world record by 0.7 seconds on Sunday night, O’Callaghan knew her training partner would be difficult to beat in a race over half the distance.

Titmus went out hard, like she did in her 400m race, and led with about 10 metres to swim.

Enter O’Callaghan. The teenager came at Titmus right in the dying stages to touch the wall first and break Italian Federica Pellegrini’s supersuit time of (1:52.98) that was untouched for 14 years.

Titmus (1:53.01) finished in second place, yet still lowered her personal best in a remarkable race. Canada’s Summer McIntosh was third, in a time of 1:53.65.

O’Callaghan, who won the 100m freestyle at last year’s world championships in Budapest and came second in this event, will remember this breakthrough victory forever.

“I was not expecting that at all. I didn’t know how I was going to race,” O’Callaghan said. “The lead-up to this has been up and down like a roller coaster, just with injury and coming off trials. To do that, I think is just incredible.

“There were tears, there was happiness, it was very mixed emotions. I’m just so proud of myself.”

Titmus was gracious in defeat as the pair stood side-by-side on the medal dais in front of both sets of proud parents.

“I have to be happy with a personal best. I thought I had a 1:52 in me,” Titmus said. “For anyone to beat me, I’m glad it’s Mollie.”

It is a unique situation for the pair, given they are both trained by Boxall, famous for his animated celebration at the Tokyo Olympics after Titmus won gold in the 400m freestyle.

“Dean is an amazing coach. He trains me for this. I can’t share any of his secrets,” O’Callaghan said.

Earlier, Short said he was ecstatic with a silver medal after not only lowering his personal best by more than two seconds, but snaring his first Australian record.

Short (7:37.76) was ahead with 50 metres to swim but fell short by 0.76 seconds as Tunisia’s Ahmed Hafnaoui powered home.

“Obviously I am moving in the right direction,” Short said. That [Hackett record of 7:38.65] was set nearly 20 years ago ... I am just glad to progress it.

“Grant is one of my heroes. I have looked up to him my whole life and even though I came second, I am over the moon with the time. I could have come third or fourth in that time and I still would have been happy.”

In April, Short surprised many by beating Hackett’s best time in the 400m freestyle before winning gold in the event on night one here, just outside Ian Thorpe’s Australian record.

With the 800m freestyle achievement also ticked off the list now, the only one remaining for Short is Hackett’s mythical Australian 1500m freestyle record of 14:34.56, set here in Fukuoka 22 years ago.

“Definitely at some point I feel [I can break it] … if not this year, definitely down the track,” Short said.

Australia (3:39.03) also picked up a silver medal in the mixed medley relay behind China (3:38.57), with a team of Kaylee McKeown (backstroke), Zac Stubblety-Cook (breaststroke), Matt Temple (butterfly) and Shayna Jack (freestyle) almost repeating the Dolphins’ first place in 2019.

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Sam Williamson missed out on a maiden world championship medal in the 50m breaststroke by just three hundredths of a second, while Kyle Chalmers posted an impressive time of 47.52 in the 100m freestyle to advance to Thursday night’s blockbuster final against the likes of world record holder David Popovici (47.66) and Great Britain’s Matt Richards (47.47).

“I think it’s the fastest heat and then semi-final that I have ever done,” said Chalmers, who has never won an individual longcourse gold medal at the world championships.

Watch the World Aquatics Championships in Fukuoka live on Channel Nine & 9Now.

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