Michael Phelps has rejoined the U.S. drug testing program, the strongest signal yet that he's planning a comeback for the Rio Olympics.

Phelps told The Associated Press on Thursday that "nothing is set in stone" though clearly he has enjoyed getting back into shape -- he's down about 15 pounds -- and working out with his former team at the North Baltimore Aquatic Club.

Michael Phelps is the winningest and most decorated athlete in Olympic history, having captured 18 gold medals and 22 medals overall at the last three Summer Games. Clive Brunskill/Getty Images

"If I decide to keep going and swim again, then I'll compete," Phelps told the AP in an exclusive telephone interview from Minneapolis, where he is attending an Arena Grand Prix meet this weekend.

"If I don't," he added, letting out a big laugh, "I guess I'll re-retire. Just don't compare me to Brett Favre."

The U.S. Anti-Doping Agency said Phelps was among the athletes who underwent doping tests in the third quarter, the period ending Sept. 30. He was tested twice.

His former coach and close friend, Bob Bowman, said Phelps actually re-entered the program near the end of the second quarter, but he wasn't tested and therefore wasn't listed that time in USADA's quarterly report. He would be eligible to compete again in March, according to Bowman.

Each week, Phelps said, he's working out a couple of days in the pool, a couple of days in the weight room, and one or two days on his core training.

"I just think he's in a place where he's feeling good about swimming," Bowman said. "If he chooses to compete, he's got some time. I like having the ability to do it. To be perfectly honest, he's not anywhere near being able to compete in a meet or anything like that. We're just getting started on improving his fitness. We'll see where that goes."