Australian swimming legend Grant Hackett returned from nine months out with a shoulder injury with a message for Welsh rival David Davies.
Hackett is expecting his rivals to improve before Beijing 2008
Hackett, unbeaten at 1500m freestyle in 10 years, was taken close by Davies and American Larsen Jensen when he claimed Olympic gold in Athens in 2004.
"I always expected them to improve but not to 14min 45sec, I think anything is possible for Beijing 2008," he said.
"I probably underestimated those guys - and I will never do that again."
Hackett, 26, aiming for his third successive Olympic gold in the event at the Beijing Olympics, believes his rivals could improve their times by at least another 10 seconds in China.
That would leave him having to break his world record of 14:34.56 to win in what could be his farewell to the 30-lap event.
"Maybe after Beijing if I feel I haven't got my best out of the 400m I might put the 1500m behind me and really focus on it," he said after competing at the Grand Prix series meeting in Brisbane.
"I really enjoy that event, it's one I think I can really bring down a lot [in time]."
Hackett has been second-best to world record holder Ian Thorpe in the 400m and his best time in the event (3:42.51) is the second fastest by an Australian.
That is more than two seconds behind Thorpe's world record (3:40.08).
Meanwhile, Barry-man Davies is recovering from surgery on an infected foot which will keep the Commonwealth Games champion out of the swimming pool for a month.
But the 21-year-old, who won 1500m freestyle gold in Melbourne in Hackett's absence, is confident he will be fit for July's European Championships in Budapest.