Williams, who won her last Grand Slam singles title in 2017 and has not competed since the 2022 US Open, can officially enter tournaments after rejoining the tennis anti-doping testing pool last year.
The American has denied that she is preparing to return to the sport she dominated for two decades, but raised eyebrows last month by deflecting questions about a comeback during an appearance on NBC's Today Show.
Macci, who previously coached Williams and her sister Venus in the early 1990s, told French publication L'Equipe that he believed she is preparing for a comeback.
"She's training with lots of sparring partners - guys," he said. "She's also played with Alycia Parks, a good friend of hers in South Florida. So I have no doubt about it (her comeback).
"I always thought she would come back from time to time to make appearances with Venus in doubles, but we're not just talking about doubles here... she's going all out.
"If she didn't think she could compete with the best and win matches, she wouldn't do it. It's 100% certain she'll dive back into competition."
Macci, 71, had no doubts about Williams' ability to handle the rigours of the tour.
"The question is whether that competitive spirit is still there," he added. "She's answering that question herself: she's coming back because she is a competitor at heart."
Williams said in August 2022 that she was "evolving away from tennis" and her team has not responded to Reuters requests for comment on her potential comeback.
Macci suggested that Williams could use Indian Wells or the Miami Open to launch her comeback next month.
"It'll be interesting to see how fit she is," he added.
"Making an appearance, playing an exhibition match or a match is one thing. Doing it again the next day and the day after that, and performing well day after day is another."