The world number seven stunned the tennis world on Wednesday by calling it a day at the age of 28, six weeks after her first grand slam triumph.
"What she did at Wimbledon was fantastic, but she will regret this decision when Wimbledon comes," Ivanisevic, unable to defend his 2001 Wimbledon title because of a shoulder injury, said in a statement released by the ATP on Thursday.
"There is nothing like playing at Wimbledon as 'Wimbledon champion'," added the big-serving Croat, who retired from the sport in 2004 after losing to Australian Lleyton Hewitt in the third round of the grasscourt grand slam on Centre Court.
Former men's world number four Forget said the decision announced after a second-round defeat by Romanian Simona Halep at the Western and Southern Open in Cincinnati sounded too hasty.
"Marion is a very smart girl, she is so dedicated about the sport that I'm always very cautious about someone's quote right after a defeat," the former French number one said in the same ATP statement.
"I hope she is going to change her mind. You don't want to have regrets looking back. Being a professional player is such an exceptional job and you don't want to look back a few months or years later and think 'why did I stop?'."
'DON'T QUIT'
He advised Bartoli to simply take a break rather than pack up her rackets for good.
"Don't rush it, just don't rush it," he said.
"Take time, go to the beach for a few days, go running in the park, just get your head together ... if you don't want to play the next week, just don't play it, if you want to miss the U.S. Open, fine, but just don't quit.
"Don't take such a radical decision only a few weeks after winning the biggest tournament in the world."
French tennis federation president Jean Gachassin, who was not aware of Bartoli's decision before she made it public, said he had not lost hope of persuading her to reverse her decision.
"Some champions have come back like (Kim) Clijsters or (Justine) Henin and I hope I will make Marion change her mind in the coming months," he told French broadcaster L'Equipe 21.
Former world number one Clijsters won the U.S. Open twice in 2009 and 2010 and the 2011 Australian Open when she returned to the sport after retiring.
"Marion has been such a fighter. I want her to continue training because I know she's going to miss tennis," Gachassin added.
French tennis coach Patrick Mouratoglou was convinced this was not the end of Bartoli's tennis career.
"Marion, hope you are already preparing your comeback ... I checked if we weren't the 1st of April when I read." he wrote on Twitter.
(Reporting by Gregory Blachier; Editing by Sonia Oxley)
