Not to mention also probably the most difficult to master.
When played at a high level, no other sport comes close to demanding the same maximum combination of aerobic and anaerobic fitness, acceleration, anticipation, agility, flexibility, endurance, speed, power, variety of movements and shots, balance, total body and hand-eye coordination, all-skill expertise, physical and mental strength, discipline, strategy, execution, precision, geometry, physics, history, emotion, respect for the opponent, honesty, and sportsmanship. The fact that tennis usually involves only a single individual with no coaching allowed during competition only amplifies the complexity. In the case of doubles, teamwork and communication are two additional necessary qualities. If you happen to be competing at the highest level, your season is essentially year-round and your opponents come from all over the world. Not to mention you are often playing in a different country each week.
This is what I mean: