Grant Hackett says illness almost made him abandon his campaign to become the oldest swimmer to make the Australian Olympic team, ahead of this week's Rio Games qualifiers in Adelaide.
But the triple Olympic gold medallist - 36 in May - said the set back earlier this year proved a blessing.
"I asked 'why am I doing this for'. I realised it was about having fun - the good thing about getting sick was it brought me back to that," said Hackett ahead of the seven-day trials starting on Thursday.
Hackett said he was "99 per cent sure" that he would retire if he missed out on making his fourth Olympic team.
He has entered the 100m, 200m and 400m freestyle at the trials but admits his best chance is making the 4x200m freestyle relay team.
Hackett made a remarkable comeback last year, making the 2015 world titles team after six years out of the pool.