Japan's multiple Olympic swim king Kosuke Kitajima has no plans to hang up his goggles, promising to come back stronger after missing qualification for this year's Pan Pacific and Asian Games.

"For the time being I won't be retiring," the 31-year-old told reporters after failing to make Sunday's 200 metres breaststroke final at the Japan Open. "I know you would like me to say I am."

The 31-year-old in 2008 became the first man to do the 100 and 200 metres breaststroke double at two Olympics, but after also missing out in the 100 in the Tokyo event, his future was immediately plunged into doubt.

"I haven't been taking it flat out this season and last," Kitajima told Japan's Nikkan Sports after 22-year-old Yasuhiro Koseki completed the double in both events.

"It's about how to approach swimming in a serious way, but I'm not retiring. I will have to begin training earlier (next season) and start from scratch."

But Kitajima did admit he could not go on forever.

"Time's running out," he was quoted as saying by local media on Monday.

Koseki, who overpowered 200 metres world record holder Akihiro Yamaguchi to win Sunday's four-lap race, grew up idolising Kitajima, watching videos of his golden doubles at the 2004 Athens and 2008 Beijing Olympics.

But Japanese media were already turning the page on one of the country's most successful athletes, referring to the "Post Kitajima" era and hailing the muscular Koseki, who stands 188-centimetre (6 foot 2) tall, as a "jumbo".

Kitajima insisted he would return, but Japan's most-decorated swimmer still had the grace to give the young pretender some advice after Thursday's 100 metres, telling him: "You can swim faster, give it everything in the 200 too."

After failing to make the cut for Japan's team at August's Pan Pacific competition in Gold Coast and the Asian Games in Incheon, which begin in September, Kitajima said he needed go back to the drawing board.

Kitajima's success and rise to fame in Japan is the stuff of legends, spending lengthy spells in an oxygen chamber as he battled a string of injuries to peak at successive Olympics.

As Japan's media hailed Koseki -- who spent his formative years studying Kitajima's stroke and famed leg kick -- the former king promised he was not done.

"The world and Olympic titles are both very attractive," said Kitajima. "That won't change. I've already done it though, so there is that inner conflict."

(For more sports news and updates, follow Ahram Online Sports on Twitter at @AO_Sports and on Facebook at AhramOnlineSports.)