"I believe in 'Sascha' who is still very young. I think he will become a great player," Lendl said in a statement released to AFP through his agent.
"But the problems outside the sport which he has at the moment prevent me from working in the way I would like to. I wish him the best for the future."
The relationship between the two men had deteriorated recently, with 22-year-old Zverev criticising what he claimed was a lack of involvement from Lendl.
"Sometimes I go on the court to practice for two hours but for half an hour, he will tell me how he played golf yesterday," the German player told a news conference on the sidelines of the Hamburg ATP tournament where he is playing this week.
"In addition to that, Lendl has a new dog with whom he spends a lot of time."
After his defeat in the first round of Wimbledon earlier this month, Lendl advised Zverev to "focus more on tennis".
"As far as my work ethic is concerned, I'm pretty confident that my track record speaks for itself," said Lendl, who won 94 ATP tournaments in his career.
Under his guidance, Zverev won the ATP Finals in 2018, easily the most prestigious victory of his career.
Lendl, 59, previously worked with three-time major winner Andy Murray.
On Friday, Zverev rallied from a set and a break down to defeat Serbian Filip Krajinovic 2-6, 7-5, 6-2 after two hours and minutes to make the semi-finals in Hamburg.
"The crowd helped me. The crowd gave me energy. The crowd got me through that one," Zverev said.
Meanwhile, Russia's world number 78 Andrey Rublev recorded the biggest win of his career by knocking out top seed and two-time Roland Garros runner-up Dominic Thiem 7-6 (7/3), 7-6 (7/5).
It was only the 21-year-old's second Top 10 win, following on from his victory over then ninth-ranked Grigor Dimitrov at the 2017 US Open.
"In my opinion, he is the best player on clay after Rafa (Nadal)," Rublev told atptour.com.
"So, of course to have a match like this is something special and for sure I hope that it will give me the confidence and I will keep working and keep improving."