GOLD COAST (AFP) - US superstar Michael Phelps came out of retirement because he believes he can swim faster than ever before, his coach Bob Bowman said on Wednesday.

Phelps, 29, admitted he was feeling "rusty" after some elementary errors at this month's US Championships, where he finished winless.

But Bowman said Phelps was driven by the conviction that he can exceed the performances that brought him 18 Olympic titles, the most achieved by any athlete.

"That's the reason he (Phelps) came back," Bowman said at the Pan Pacific Championships, Phelps's first international meet since ending his 20-month retirement in April.

"If he didn't think he had the chance to get faster I don't think he would have." Bowman, who is also the head coach of the American men's team, said Phelps needed to "get back into racing speed and the pressure - he just needs more experience with that".

Phelps said he was expecting improvements at the Pan Pacs, on Australia's Gold Coast, where he will be racing against fellow American star Ryan Lochte.

Phelps, who has entered five events and three relays, said he had gone back to basics ahead of his return to the international stage.

"In the past I've usually had pretty good success coming off a meet and going right into another one," he told a media conference. "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."

He added that he just needed more race experience. "I hate to lose but I know it is not going to come back overnight."

Phelps said he would rather have the problems he has been experiencing now than further down the road at next year's world championships in Russia and the 2016 Rio Olympics.

"I guess I'm kind of rusty on judging speed going into my walls," he added. "That was one of the biggest things I had trouble with at nationals - my flip-turns and my open turns.

"Those are the kinds of mistakes you have when you're an age-group swimmer. I probably haven't had enough experience over the last year.

"It's something Bob (Bowman) and I have spent more timing working on in training camp, to judge the walls a little bit better and to judge them at a high speed."

Lochte, who has endured a difficult year, missing training time after tearing a ligament in his knee, said he expected tough opposition at the Pan Pacs.

"Team USA didn't do as well as we thought we were going to do at our nationals. But we had two weeks to prepare after nationals for this meet," he said.

"It's going to be good. You can't exclude anyone. It's going to be a race in every event." Lochte said that in the 200m individual medley, in which he is world champion, the competition had become intense.

"It's no longer just me and Michael (Phelps). There's other people. It's definitely become a bigger event, and bigger race."

The Americans will be up against hosts Australia, Canada, Japan and 14 other nations on the Gold Coast.