Swimmer Michael Phelps, sprinter Tyson Gay and gymnast Shawn Johnson all face cut-throat competition in the coming weeks as they bid to secure spots on the United States squad for the Beijing Olympics.

While Phelps is recognised as one of swimming's all-time greats and in many respects has already become the poster-boy for the Beijing Games, it accounts for little at the US Olympic trials with its "win and you're in" qualifying criteria.

Phelps has targeted Mark Spitz's Olympic record of seven gold medals at a single Games in Beijing but could see his quest end in an Omaha, Nebraska pool early next month where he will have to qualify for each and every event alongside other hopefuls.

Neither will there be free passes issued to Gay, the 100 and 200 metre world champion or Johnson, the reigning women's all-round champion, who like every other American athlete must book their Beijing ticket at their respective Olympic trials.

"It's a format that is unique and straightforward," USOC spokesman Darryl Seibel said. "It rewards athletes who are performing at their best and that is part of the reason that the trials are scheduled when they are.

"There is no doubt it helps prepare athletes for the Games having weathered the intensity and pressure of the trials.

"They go in (to Beijing), tested, prepared and confident."