(CNN) -- Is the greatest Olympian of all time making a comeback?
American swimmer Michael Phelps, winner of a record 22 medals at the Games --18 of which were gold -- fueled speculation that he could stage a comeback by undergoing two drug tests over the past five months.
Phelps, who seemingly brought the curtain down on his glittering career at the end of London 2012, has been tested twice by the United States Anti-Doping Agency (USADA) since July 1.
By entering USADA's testing program, the 28-year-old is eligible to compete from March 2014 onwards.
Swimming's global governing body FINA requires that athletes who intend to return to competition after announcing their retirement should be tested "at least nine months before he or she expects to return to competition."
Rumors of a possible return for the "Baltimore Bullet" are sure to excite swimming fans, with Phelps himself refusing to rule out a comeback.
"Nothing is set in stone," Phelps told the Associated Press (AP). "If I decide to keep going and swim again, then I'll compete.
"If I don't, I guess I'll re-retire. Just don't compare me to Brett Favre," he added, referring to the famed NFL quarterback who made a string of comebacks.
According to the AP article, Phelps is training with his former team at the North Baltimore Aquatic Club and has shed 15 pounds.
Coach Bob Bowman insisted that his return to the pool does not mean he will be lining up at the Rio Olympics in 2016.
"It's premature to say that," Bowman told USA Today. "What we're doing is kind of letting him have his options.
"He came back this fall and started to do some training with the group, mainly just to get in shape. He just felt like he was not fit. He wasn't.
"He's occasionally been training. He's picked it up a little more. We were just thinking about it, and I said, 'You know, you're getting in pretty good shape, maybe you want to swim in a meet?' He said, 'Well, maybe at some point.' "
If Phelps was to return to action in March, his first major international meet wouldn't be the Olympics but the 2015 World Championships in the Russian city of Kazan.
Phelps has claimed 33 World Championship medals dating back to 2001, including 26 golds.