Altitude changes the tactics more than it changes the ball

Thin air is usually explained as a ball that flies further, and the harder consequence is that half a player's shot selection stops working.

Thin air means less drag. Less drag means a ball that travels further for the same swing and, more importantly, curves less. That second part is where matches at altitude are won and lost, and it gets far less attention than the first.

Topspin does not work by magic. It works because a spinning ball moving through air experiences a pressure difference that pushes it downward, which is what lets a player hit hard, high over the net, and still land it inside the baseline. Reduce air density and you reduce that force. The ball still spins. It just does not dive the same way. A player whose entire margin comes from heavy topspin arrives at a high-altitude event and finds the shot he has hit ten thousand times landing a metre long, and no amount of extra spin fully recovers it because the mechanism itself is weakened.

The adaptation is not to swing softer. It is to swing flatter and lower over the net, which is a different shot with different footwork requirements, and it takes more than a week of practice courts to make automatic.

Serving flips the same way. A kick serve relies on air to make the ball climb and jump. In thin air the jump flattens out, so the safe second serve stops being safe. Meanwhile the flat first serve gains real speed, so aces go up and rallies get shorter. A big server who cannot rally is a much better player at altitude than at sea level, and a grinder who lives on margin is a much worse one.

This is why I have never been comfortable with treating results at high-altitude events as directly comparable to results elsewhere. It is not a small surface variation. It is a change to the physics that a specific style depends on.

Humidity works in the same direction and surprises people, because humid air is slightly less dense than dry air, not more. The feeling of a heavy, slow ball on a muggy day comes mostly from the ball absorbing moisture and from the court, not from the air itself.

Temperature is the third lever, and the biggest one at most venues. Warm air is thinner and a warm ball is bouncier.

Same tournament, same court, morning and evening. Two different games, and the schedule decides which one you get.