Tsitsipas, the 2021 French Open and 2023 Australian Open runner-up, was forced to retire from his Wimbledon first-round match while trailing 6-3, 6-2 to French qualifier Valentin Royer on Monday due to a back injury.
The 26-year-old, who said he had no answers to his ongoing fitness problems after his elimination, appointed Croatian Ivanisevic as his coach in May after a string of disappointing results at the Grand Slams.
Tsitsipas, a former world number three, has reached only one quarter-final in his last nine Grand Slam tournaments.
"It's simple and it's not simple. I've talked to him a lot of times. If he solves some things outside of tennis, then he has a chance and he'll return to where he belongs, because he's too good a player to be out of the top 10," Ivanisevic told Serbian network Sport Klub after Tsitsipas' exit.
"He wants to but he doesn't do anything. All 'I want, I want', but I don't see that progress... I was shocked, I have never seen a more unprepared player in my life. With this knee, I am three times more fit than him. This is really bad."
Ivanisevic, who won Wimbledon as a player in 2001, helped Novak Djokovic claim nine of his 24 Grand Slam titles before leaving his team in March last year. He then had a short stint with Kazakh world number 11 Elena Rybakina this season.