Michael Phelps will not compete at the world championships but the 18-time gold medallist will attempt to add to his Olympic legacy by competing at next year's Rio Games.

Since coming out of retirement two years ago there had been widespread speculation Phelps, considered the greatest swimmer of all-time, was targeting a comeback at Rio although the 29-year-old American did not confirm that until 15 April.

"You guys heard it here first. Yeah, look it's a big surprise," Phelps told the media with a laugh and smile

Phelps returns to competition this week at the Arena Pro Swim Series in Mesa, Arizona, after serving a six-month suspension imposed by USA Swimming for a drunk driving conviction.

He was also banned from the world championships in Kazan as part of the punishment, though last month USA Swimming executive director Chuck Wielgus said they may consider including him in the team, but they have since closed that door.

"It's obviously very challenging and hard for me to not be able to participate at world championships," Phelps said.

"I will be in full support of everything they do over there, and I wish them all the luck in Kazan."

Phelps will launch his latest comeback on Thursday when he races in the 100-metres butterfly.

He is entered in a total of five events including the 100 backstroke and 400 freestyle on Friday and 200 individual medley and 100 freestyle on Saturday.

The meet has attracted some of the top US swimmers, including Phelps's chief rival Ryan Lochte, who is entered in the same five events.

It will be familiar surroundings for Phelps, who ended his two-year retirement in 2014 at Mesa, which sparked the speculation he was considering an Olympics comeback.