At 29 years of age, most people could only dream of retiring from professional life.

But Michael Phelps, considered as one of the greatest Olympians of all time, has decided to comeback in the sport, saying retirement from the thing he loved most was "pretty boring."

Phelps is set to compete in the U.S. National Championships in Irvine on Wednesday.

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"I would literally do nothing," Phelps said of his short-lived retirement.

"It's good to have some structure back in my life ... That's something I need," Phelps added.

Phelps will be competing in four events, the 100-meter freestyle, 100 butterfly, 100 backstroke and 200 individual medley at the said nationals.

Ryan Lochte, a long-time rival of Phelps since the 2004 Athens Olympics, expressed his enthusiasm of Phelps' comeback.

"I'm happy he's back," Lochte said.

Lochte, a five-time Olympic Gold medalist, will also be competing in the nationals in six events, four of which Phelps are competing in.

This has given Lochte some motivation as Phelps is "the toughest racer I've ever had to go up against," he said.

Bob Bowman, Phelps' longtime coach, said he looked more relaxed, and he did not feel any pressure.

"He's doing it because he wants to, not because he has to. That's the big difference," said Bowman.

Lochte and Phelps are good friends outside of the pool. Lochte even made a bet that Phelps will be back before the 2016 Rio Olympics.

"He owes me money," Lochte added.

Aqua Sphere CEO Don Rockwell, whose company recently partnered with Phelps, had no idea that he would come out of retirement.

"We would have been happy to have him," Rockwell said.

Both Phelps and Bowman had signed deals with Aqua Sphere to develop a new class of swimming products including expanding efforts on water-safety.

TagsBob Bowman, Ryan Lochte, Aqua Sphere, Olympics, swimming