Michael Phelps smiles during a workout in San Antonio, Texas, ahead of joining the US team in Rio.?Reuters

Superstar swimmers Michael Phelps and Katie Ledecky will both represent the United States in the 4x100-meter freestyle relay at the Rio Olympics, USA Today reported on Monday.

Neither of the American team's best-known swimmers, with 18-time gold medalist Phelps seeking to add to his collection of 22 medals, qualified for the individual freestyle events at the national trials.

Five-time Olympian Phelps, the most decorated athlete of all time, is set to compete in three individual events - the 100- and 200-meter butterfly and the 200 individual medley.

The relay lineups, a closely-guarded secret, do not have to be submitted until the night before the race, but it would be no surprise for the pair to be included in the 4x100.

Both US relay teams are likely to finish in the top three, which would give Phelps and Ledecky their first medals of the Games.

USA Today, in a report citing an unnamed source with direct knowledge of the US swimming team's plans, said Phelps will swim in the men's relay final on Aug 7.

It said Ledecky, the 19-year-old tipped for gold in the 200, 400 and 800 freestyle, was expected to take part in the preliminary heats on Aug 6 in what would be her first competitive dip in the Rio pool.

Swimmers who take part in the preliminary heats, but not the final, also get medals.

It is common for the finals lineup to differ from the preliminaries because it saves the top swimmers for the medal showdown and allows more to share in the glory.

Ledecky finished seventh in the individual 100m freestyle at last month's US trials in Omaha, Nebraska while Phelps did not enter.

Nathan Adrian was fastest in the trials, with Caeleb Dressel second and Ryan Held third. Others in the mix are Anthony Ervin, Jimmy Fiegen and Blake Peroni.

However, Phelps showed impressive speed at the team's pre-Games training camp in Atlanta.