Michael Phelps will become the most successful athlete at one Olympics by winning eight gold medals in Beijing, according to Mark Spitz - the man he is bidding to overtake.

The 23-year-old from Baltimore begins his record bid in the heats of the 400 metres individual medley on Saturday after narrowly failing to match fellow American swimmer Spitz's haul in Athens in 2004.

Spitz claimed seven titles and seven world records in Munich in 1972 before Phelps won six gold and two bronze medals four years ago.

But Spitz, who claimed nine Olympic golds in all, having won two in Mexico City in 1968, anticipates Phelps will top the podium in all eight events he has entered in the XXIX Olympiad having improved in the last four years.

"He's definitely a better swimmer now," said Spitz.

"I'd say there's maybe a three or four percent chance that he's not going to win eight gold medals and it will only be because he might get sick or for some unforseen situation."

Spitz predicts the 100m butterfly, with the final on the penultimate day of competition, will be Phelps' toughest obstacle.

"The 100 fly will be one of his hardest races," added Spitz.

"It's the last individual event he swims and he's going to think whether he should swim, like I did.

"I'd rather get six for six wins, than six wins and a third - he's already done that.

"And the world record holder is swimming there and Ian Crocker could all of a sudden go I'm going to have the race of my life again'."

Phelps' bid could also come unstuck in the relays, with France, Australia and South Africa capable of winning, according to Spitz.

"He's got several hurdles he's got to go through - it's not going to be easy," he added.

Spitz will be in Detroit during the Games after not receiving an invite to take part in the official proceedings.

"I would've loved to have been there, but in light of the fact that this guy is trying to break my record, for me to be there anonymously, to sit in the stands and watch it, I'd be really uncomfortable about doing that," said Spitz.

"If I was a part of the festivities, an award ceremony, that would be a lot more enjoyable for me, but I was never asked."