Whenever Alexander Zverev, 29, plays competitive tennis, he has two opponents to tackle. The first, obviously, is the man on the other side of the net. The second is the medical condition he was diagnosed with at the age of four — Type 1 diabetes.

Type 1 diabetes is a condition in which the pancreas produces little or no insulin, the hormone required to regulate blood sugar levels. Unlike Type 2 diabetes, it is not caused by lifestyle and currently has no cure. Patients have to rely on externally administered insulin, constantly monitor their glucose levels and carefully manage their diet and physical activity throughout their lives. Now imagine having to serve, volley and sprint across a tennis court for several hours while managing all of that in an intensely competitive environment.