Novak Djokovic tells the Wall Street Journal that to recover, he’s been sitting in a hyperbaric chamber that is said to simulate high altitude and compress the muscles at rhythmic intervals.

The machine, which is made by CVAC Systems and called the CVAC Pod, costs $75,000. The company claims that spending up to 20 minutes in the pod three times a week can boost athletic performance by improving circulation, boosting red-blood cells, removing lactic acid and maybe aid in stem-cell production.

"I think it really helps—not with muscle but more with recovery after an exhausting set," Djokovic said. "It's like a spaceship. It's very interesting technology."

CVAC Systems CEO Allen Ruszkowski claims that the technology may be twice as effective at helping the body absorb oxygen as blood doping, which is banned as a performance enhancement. Other athletes are said to use the pod. Ruszkowski said they do so "because they feel it's a competitive advantage."

During the U.S. Open, Djokovic is said to be staying in the New Jersey home of former pro Gordon Uehling, who owns such a machine. Another former touring pro, Geoff Grant, tried the machine and said that that some of his current clients "say it's like a drug. It's weird—it's definitely something from the future."