American Phelps accounted his second retirement at the age of 31 after ending the Rio Games last August as the most decorated Olympian of all time with an astonishing haul of 28 medals over three Olympics, concluding with five golds and one silver in Brazil.
Phelps has indicated he would like to focus on coaching, but there remains the prospect of increasing his haul to 30 medals if he opted to return for a fourth Games.
Spitz - who won seven golds at the 1972 Munich Olympics - feels Phelps could still compete in Tokyo at the age of 35 if he reduced the amount of events he was targeting.
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"You never know. He may think it's easier to swim one event rather than six or four," said Spitz.
Spitz is in Hungary as an analyst for Brazilian channel SportTV and has been continually asked this week who could replace Michael Phelps as swimming's next male superstar.
"There is a big absence with Michael. Who can take his place? Well, it took 36 years to find somebody to take my place," the 67-year-old said.

Michael Phelps to turn his attention to coaching after Rio 2016

It will be safe to say that it will take more than one year after the Olympics to find the next Phelps.
Spitz believes Sarah Sjostrom, Sweden's sprint sensation, can swim even faster after she became the first woman to go under 52 seconds for the 100 metres freestyle.
He was left in awe of Sjostrom, who dipped under 52 seconds when leading off Sweden in the women's 4x100m relay at the FINA World Championships on Sunday.
Spitz himself became the first man to break 52 seconds in 1970 before posting 51.47 seconds in the blue riband event ahead of his Munich tilt.
"In the 47 years it has taken a woman to go as fast as a man there have been three of four generations that have come and gone since I retired," said Spitz.

Rio 2016 Phelps

Image credit: Eurosport

There will always be a comparison where a woman can be as fast as a man but it's now testament to new types of training and cross-training. It's exciting.
Sjostrom also became the first woman to win four gold medals in the women's 100m butterfly - her first coming in 2009 as a 15-year-old - while the garlanded 23-year-old has the 50m in both freestyle and butterfly still to come.
Sjostrom began her 100m individual bid on Thursday by easing into semi-finals in 53.01 seconds.
Spitz blitzed to Munich gold in 51.22 and admits Sjostrom has the capability to make further inroads on his Olympic mark.

Olympic gold medal swimmer Mark Spitz attends swimming semifinals and finals on Day 6 of the Rio 2016 Olympic Games at the Olympic Aquatics Stadium.

Image credit: Eurosport

"Sarah has raw speed and isn't afraid to use it at the beginning of her races," said Spitz, who was also a freestyle and butterfly specialist.
"There are no tactics with her and she's out in front at the start."
Spitz, who retired aged 22, says the sprint calibre of 20-year-old American Caeleb Dressel could lift him up the ranks.
"He does 50 and 100 fly, 50 and 100 free and swims in the relays. Right there you have one person who can swim in seven events," Spitz said.
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