Last month, at Australia’s Olympic swim trials in Brisbane, former 200m freestyle world champion Mollie O’Callaghan broke her own world record. There was just one problem – teammate and rival Ariarne Titmus went even faster. O’Callaghan, only 20, just about kept it together on camera in the post-race interview. But once the broadcast was over, the tears flowed.

There were more tears on Monday evening in Paris. Only they were happy tears, tears of jubilation and tears of relief. O’Callaghan may have lost the battle last month, but at a packed La Défense Arena she won the war. In one of the most anticipated encounters of the meet, O’Callaghan got her revenge, winning her first individual Olympic gold in a frenetic four-lap battle with Titmus. It is the first time in 20 years that Australian swimmers have gone one-two at the Olympics, as the pair followed in the footsteps of Ian Thorpe and Grant Hackett.

The starting gun went. O’Callaghan was slightly quicker off the blocks. And then they raced. Hong Kong’s Siobhan Haughey touched first at 50m; by the second turn Titmus had moved up to second, while O’Callaghan was back in fifth. Then the Australians came to the fore. By the final turn, they were neck-and-neck, before O’Callaghan exploded in the final 50m. She had touched in third at the last turn, before finishing with an Olympic record – bettering Titmus’s time from Tokyo.

O’Callaghan looked up, stunned. The pair embraced in the water, and then again on land. The Australians held their hands aloft together in a touching display of camaraderie.

“I can’t explain it, there are no words to explain,” an emotional O’Callaghan said afterwards. “I’ve got to stay steady and calm, I have the 100m free tomorrow. I need to stay collected, calm and cool for this week. I’m swimming for the country. I’ll let it settle in at the end of the week.

“I had a few tears, and a few tears with Arnie,” she admitted. “It’s just great to race alongside my teammate and be on the podium with her.”

Tears also flowed from Titmus. “These are happy tears,” she said after winning silver. “It’s really hard to hold your emotions in, in these situations. I know what it’s like to be Olympic champion – I’m honestly happy for her, I’m really happy to be on the podium. I had nothing to lose tonight.”

The duel between the two Australians had been made all the more intriguing because the pair train together under the watchful eye of coach Dean Boxall. In the hour before the race, Boxall gave both of his athletes a plan to beat the other. But only one could emerge victorious.

It was a battle between speed and endurance. O’Callaghan is a two-time world champion in the 100m freestyle and a strong favourite in the two-lap dash later this week. Titmus’s pet events are the 400m and 800m. Indicative of this divide, and the cordial but not exactly close relationship between them, the pair train in different groups within Boxall’s program – Titmus is part of the endurance squad, O’Callaghan swims with the sprinters. On Monday, O’Callaghan’s speed in the final lap was just unbeatable.

Day three in the pool began with the women’s 400m individual medley, with Australia’s Ella Ramsay battling valiantly for fifth in a race won empathically by Canadian prodigy Summer McIntosh. The 20-year-old follows in the footsteps of her father, Heath Ramsay, who swum for Australia at the Sydney 2000 Olympics; Ramsay is considered a rising talent in the Dolphins program. The medal events continued with the men’s 200m freestyle – Australia’s Max Giuliani finished seventh, as Romanian sensation David Popovici touched first.

In the women’s 100m backstroke semi-finals, Australia’s Kaylee McKeown qualified second fastest ahead of Tuesday night’s medal race. The 23-year-old is the reigning Olympic champion in both the 100m and 200m backstroke; in Paris, she will also contest the 200m individual medley. There is every possibility she could walk away with three gold medals.

But in the 100m she will face stiff competition; current world record holder Regan Smith of the United States was fastest in the first semi-final, two one-hundredths of a second faster than McKeown. Fellow Australian Iona Anderson also moved into the final with the fourth fastest time.

On Monday night, the battle royale was left until last. Titmus versus O’Callaghan. Olympic champion versus world champion. Australian versus Australian. Past world record holder versus current holder. It did not disappoint.