Olympic legend Michael Phelps returned to competition because he believes he can swim faster than ever, the American's coach Bob Bowman said.
Phelps, 29, who won a record 18 Olympic titles from 2004-2012 and 22 medals in total, announced his comeback in April.
"If he didn't think he had the chance to get faster, I don't think he would have come back," Bowman said.
Phelps races in five events and three relays at the Pan Pacific Championships in Australia, which begin on Thursday.
The four-day meet at the new Gold Coast Aquatic Centre in Southport is his first international event since ending his 20-month retirement and it is regarded as key preparation for next year's World Championships in Russia and the 2016 Rio Olympics.
Phelps qualified for the US team after two second-placed finishes at the trials earlier this month in Irvine, California.
|Michael Phelps facts and figures|
|58 gold medals: 18 Olympic, 27 World Championship, 13 Pan Pacific|
|100m butterfly world record: 49.82 seconds, Rome, August 2009|
|200m butterfly world record: 1 minute 51.51 seconds, Rome, July 2009|
|400m individual medley world record: four minutes 3.84 seconds, Beijing 2008|
After coming seventh in the 100m freestyle, he was beaten by a fingertip in one of his signature events, the 100m butterfly, and finished sixth in the 100m backstroke.
But he staged an exciting battle with Ryan Lochte in the 200m individual medley and finished just 0.05 seconds behind the world record holder in the event in which he has won three Olympic golds.
Asked about his form, Phelps said: "I have been working on the basics that hopefully make a big improvement with my freestyle stroke.
"At the nationals, I missed a couple of walls, I jammed a couple of walls. Those were the big things that I guess I forgot how to do. With some extra practice, I hope it's going to pay off."