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.
"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."
By subjecting himself to drug testing, Phelps has given himself plenty of time to go through an entire season before the next major meet, the 2015 world championships in Russia, an important steppingstone to the Rio Games the following summer.
FINA, the world governing body for swimming, requires an athlete to be tested for at least nine months before taking part in sanctioned events.
"This may not go anywhere," Bowman said. "We don't have an event picked out or anything like that. There's no grand scheme. It just sort of makes sense that he can make some choices if he wants to."
Further stoking speculation about a comeback, Phelps turned up at the meet in Minneapolis and even did some laps in the diving well.
"I just splashed around a little bit," he said. "The guys are swimming laps around me. But at least I'm exercising and trying to get back into some respectable shape.""
Phelps attended the world championships in Barcelona this past summer, but he seemed even more pumped about being on hand for one of the more mundane events that fall between the Olympic years.
"I just wanted to come up here and see how a meet was," he said. "This is kind of like a normal meet. Barcelona was not a normal meet. That's a big boy meet. Just being around this is exciting for me. It really has been a part of my life ever since I was a kid."
Of course, it's a little different when you're not competing.
"I said to Bob, 'Man, this takes forever.' These meets take so long if you're not swimming," Phelps said. "Man, it's so hot on the pool deck. It's absolutely brutal. It's a hell of a lot cooler in the water."
That sort of talk is sure to validate the notion that a Phelps comeback is more a matter of when, not if. His return would surely be welcomed by the entire sport and even those who only follow swimming during the Olympics.
"I don't think we look at it as bad news," said Chuck Wielgus, the executive director of USA Swimming. "I want Michael to do whatever he thinks is best for Michael."
Phelps is the winningest and most decorated athlete in Olympic history. He captured 18 gold medals and 22 medals overall at the last three Summer Games, shattering the previous marks. He is best known for breaking Mark Spitz's record for a single Olympics by winning eight gold medals in Beijing in 2008.
But his appeal goes far beyond the pool deck. Phelps' recognition factor has often matched athletes from far more prominent sports, such as NBA star LeBron James, a truly impressive accomplishment for a swimmer.
Phelps retired at age 27 after winning six more medals at last summer's London Olympics, adamant that he had no intention of competing again. He had long said his goal was to retire from swimming before he turned 30.
"Sure, I could come back in another four years. But why?" he said last December, after beating out James as the AP's male athlete of the year. "I've done everything I wanted to do. There's no point in coming back."
But, as speculation swirled about a possible flip-flop, Phelps softened his stance this past summer. He told the AP during the world championships, "I don't know what's going to happen in the future. I don't know what's going to happen tomorrow."
Now, he's moved even closer to a comeback.
"If I do really start getting excited and wanting to do it, I can make that choice," Phelps said Thursday. "If not, at least it's something we can say we were prepared for."
Phelps will be 31 at the time of the opening ceremony for the Rio Games - not that old, really, for today's top-flight swimmers, who have more opportunities to cash in on their success and can extend their athletic careers well into their 30s.
Heck, Dara Torres was 41 when she won three silver medals in Beijing.
Phelps doesn't need the money, of course, having earned tens of millions of dollars in endorsements during his career, and he remains a marketable name, even in retirement. If he does come back, it will likely be another case of an athlete who simply missed the thrill of competition, the day-to-day grind of proving himself against other top swimmers.
After London, Phelps talked longingly about sleeping late and doing what he wanted without dealing with the often-brutal demands of his sport. He opened a chain of swim schools and planned to work more extensively with his foundation, which is devoted to water safety. He had hoped to stoke his fierce competitive side by getting serious about golf, taking part in a television series with famed coach Hank Haney.
Phelps even joked that golf, which is returning to the Olympic program in 2016, was the only sport he'd possibly compete in at Rio.
Now, he's moving back toward his true love.
"There are a lot of things that really excite me ... that get me motivated," he said. "But I swam for 20 years. That's something that's going to be very, very hard to top."