Michael Phelps prepares to enter the pool prior to swimming laps during a practice session, April 15, 2015, in Mesa, Arizona, US. [Photo/IC]

Phelps' longtime coach, Bob Bowman, acknowledged his star swimmer's second arrest for drunken driving made it "pretty hard to convince me that anything was going to lead us back to something that we'd be proud of."

Phelps called Bowman a few times from rehab, which the coach described as "really weird" because he typically never heard from Phelps away from the pool. Bowman visited Phelps during treatment, still skeptical that the swimmer was there for any reason other than he had to be. But Bowman left thinking differently.

"I just had no doubt that he had changed in a way that was really meaningful," Bowman said. "He was completely different and it's been that way every day since he's been back."

Phelps pleaded guilty after his first drunken driving arrest in 2004. Five years later, a photo was published of him using a marijuana pipe at a party, which drew a three-month suspension and caused some sponsors to drop him.

Phelps knows not everyone will believe he's contrite and has changed.

"If somebody doesn't believe the words that are coming out of my mouth that's their choice," he said. "I, of course, would like to show everybody in the world that I am in a different place and I am much better than I ever have been. I understand that's going to take a lot of time. This week will be the first week that I can start that."

Phelps will be competing in Mesa for the first time since the Pan Pacific Championships last August. The Arizona meet was where Phelps made his comeback a year ago after an extended retirement following the 2012 Olympics.

He isn't sure what to expect, having only recently returned to the kind of hard training he did before the 2008 Beijing Games, where he won a record eight gold medals. Phelps is falling back on the mentality he used for much of his career.

"I would just get up and swim as hard as I could and leave it in the water," he said. "That's what I'm going to do here. I know I'm more prepared now than I was at this time last year."

His first event is the 100-meter butterfly on Thursday, when he'll compete against longtime rival Ryan Lochte. Phelps was wearing a shirt from his new "MP" brand that launched Wednesday. He'll be wearing his new Xpresso suit for the first time after he and Bowman collaborated on its manufacture with Aqua Sphere.

Phelps is surrounding himself with familiar faces this week, including his fiancee Nicole Johnson and family members.

"I'm a lot happier and in a lot better state and in a lot better physical shape than I really have been in a long time," he said.