I did this for the first time and noticed that the balls moved much faster and bounced much higher and you also become winded more quickly. Topspin forehands were bouncing 7 feet high instead of 5 feet using new full pressured balls. Balls were often sailing long due to less air to slow them down. Rallies were generally short because of this which was good since you become more tired quickly chasing balls with less oxygen for your lungs. I played at West Yellowstone and also Jackson Hole which are both over 6000 feet above sea level. It took awhile to adjust my game to the conditions but found hitting high kick serves to my opponents backhand was a good play.