As tennis’ US Open gets underway, The Wall Street Journal offered a glimpse into what makes Serbian tennis star Novak Djokovic so spectacular. Djokovic spends time inside of a $75,000 CVAC pod, a sort of egg-shaped pressurized oxygen chamber, the he believes helps his fitness and footwork.

There are reportedly only about 20 in the entire world.

Through simulating high altitude conditions, the CVAC pod can help improve circulation, vision, reaction times, endurance and power. It appears to have paid off for the world’s top-ranked tennis player. “It’s like a spaceship,” Djokovic said.

Djokovic’s antics are well-documented and some have scoffed at his stunts. But whatever Djokovic is doing seems to be working.

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