What is harder tennis or soccer?

Both are hard/easy in different ways.

Soccer (also called football in most parts of the world) requires someone to have incredible stamina to run non-stop for long periods of time, and it requires adept skill at handling a ball with the feet. Strategically, soccer is difficult because you have to coordinate a team of people who are in constant motion. In this sense, it is similar to Basketball--except that time-outs are more common basketball and the court is smaller in basketball than is a field in soccer. The game requires focused and diligent teamwork at all times.

Baseball requires a combination of the ability to sprint and fine-tune hand-eye coordination. Batters and pitchers alike need to have physical strength and master subtle control of the bat or ball. Catchers and pitchers need to coordinate their offense strategically based on the context at any given time. Certain positions have particular demands, e.g. fielders need to be sprinters, catchers need strong thighs, butts, and joints, etc. Baseball tends to favor players who are quite tall, as well.