If you can do meth, and still manage to play professional tennis at that level, it’s more of a testament to his talent than anything.
Most meth users end up licking the furniture in a cheap motel near the Mexican border.
I'm not a medical professional but my understanding is drugs affect people differently. The Japanese who invented it also distributed it amongst military forces in WWII to help them make that extra sacrifice for the emperor. In WWII, meth under the name pervitin was used heavily by German forces.