Alexander Zverev revealed Saturday he was told he would never be an elite athlete due to having type 1 diabetes, firing him up to prove the doubters wrong.
The German former world number two, who is on a comeback from tearing ankle ligaments at the French Open last year, was diagnosed aged three with the chronic condition where the body cannot produce insulin.
"My parents were very scared. They were very worried. Mum was crying a lot," he said on Australia's Channel Nine about when he was first diagnosed.
"A lot of parents get intimidated by a lot of doctors who say 'your kid is very limited', which is not the case.
"I always said to the doctors, 'yeah, well, I want to play tennis. That's the only thing I really care about'.
"Some of them said, "No, you have to stop ... there is no way you can be a professional athlete with this kind of illness. There is no way you can play such a hard physical sport.
"This is what really stuck in my mind, made me quite upset, to be honest. I don't think you should set any limits to kids, because I think that is just not fair to them."
Zverev, 25, has gone on to become one of the top players in the world, winning 19 ATP titles and an Olympic gold medal.
Last year, he set up the Alexander Zverev Foundation to help young people to avoid limiting themselves because of the condition.
"That was the goal of my foundation, to send a message out there that you can have a normal life," he told the broadcaster. "You can become anything you want with this kind of illness.
"There are a lot of Olympic gold medallists with diabetes. There are a lot of great footballers in Europe as well. There's really no limit to what you can do."
Zverev admitted he had struggled to accept his condition in the past and tried to hide it from the world, feeling "uncomfortable", but decided to go public last year to help others.
Now, he sometimes administers insulin during matches.
"In matches, you never saw me do a shot or anything like that ... I was going to the bathroom to do it, which is not the right thing to do because you should never be embarrassed of it," he said.