Kurtis Mathews came out of retirement and had to knock himself into shape to rekindle an Olympic diving dream.

But all the "brutal" work to get back to his fighting weight has now been rewarded with a place in the semi-finals of the 3m springboard event at the Paris Aquatics Centre.

The 25-year-old Victorian-born, Sydney-based athlete qualified in eighth place to take his place in Wednesday's 18-man semis, looking in the sort of form to achieve his ambition of making Thursday's 12-man final.

And that would mark the culmination of a remarkable comeback, which saw Mathews give up the sport after becoming a double American college champion at Texas A&M University and take up playing recreational club rugby instead.

His weight ballooned to 90kg in Texas, but a chat with an old diving friend, former world medallist and Olympian Kevin Chavez, who told him 'you've gotta come back', persuaded him to come out of retirement to try to reach Paris.

"I really did believe that I was done, but just that one conversation sort of changed my mind. Without Kevin, I really wouldn't be here. I'd probably still be playing rugby and finishing up my Masters degree," said Mathews.

"So I came back to Sydney and had 13, 14 months to prepare. I had to lose 14 kilos to get back into shape, and it was brutal. It was so bad. I mean, I'm never getting out of shape again, it's the hardest thing ever to bounce back from."

But refreshed and rejuvenated, Mathews, who earned his place in Paris by beating Tokyo Olympian Li Shixin at the Australian championships in Adelaide in June, performed excellently, scoring 399.20 points behind China's soaraway top two, Wang Zongyuan (530.65) and Xie Siyi (509.60).

There was a brief moment when he plunged towards the qualification danger zone after a poor 3.5 somersaults effort, but he responded next round with his best dive, an inward 3.5 somersaults.

"It's probably the most phenomenal experience I've ever had. I've never had that much energy in my body ever," beamed Mathews.

"It was quite difficult to feel that energy but then at the same time, calm yourself down so that you have a controlling competition. I think I did fairly well.

"Basically, this was the thing that I needed to do to make my career worth it, in my eyes - and I've just made a semi-final, so that's even better.

"My goal was to make a final, but if I don't make it through, I'd be disappointed in the short-term but I'm also okay with it because I've already achieved everything I wanted to. But to do a final would be incredible."