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Serbian tennis star Novak Djokovic hasn't earned his No. 1 ranking by taking the conventional road. There's his odd ritual of excessive ball bouncing before serves, which can break an opponent's concentration. There's his new gluten-free diet, which he's said has helped him feel stronger on the court.

But now there's something truly weird: the CVAC Pod.

Ever since last year's U.S. Open, Djokovic has been trying to improve his fitness by climbing into a rare $75,000 egg-shaped, bobsled-sized pressure chamber.

The machine, which is made by a California-based company called CVAC Systems and hasn't been banned by any sports governing bodies, is one of only 20 in the world. Unlike the increasingly trendy $5,000 hyperbaric chambers many professional athletes use to saturate the blood with oxygen and stimulate healing, the CVAC is a considerably more-ambitious contraption. It uses a computer-controlled valve and a vacuum pump to simulate high altitude and compress the muscles at rhythmic intervals.

But now there's something truly weird: the CVAC Pod.

Ever since last year's U.S. Open, Djokovic has been trying to improve his fitness by climbing into a rare $75,000 egg-shaped, bobsled-sized pressure chamber.

The machine, which is made by a California-based company called CVAC Systems and hasn't been banned by any sports governing bodies, is one of only 20 in the world. Unlike the increasingly trendy $5,000 hyperbaric chambers many professional athletes use to saturate the blood with oxygen and stimulate healing, the CVAC is a considerably more-ambitious contraption. It uses a computer-controlled valve and a vacuum pump to simulate high altitude and compress the muscles at rhythmic intervals.

Djokovic is so convinced that the pod helps his game that during the U.S. Open, which starts Monday, he's staying (for the fourth year) with a wealthy tennis-trainer friend in Alpine, N.J. who keeps one of the machines on his property.

Djokovic has never mentioned the pod publicly before. He acknowledged using it for the first time last week during a sponsor event in New York after he was asked about it for this article. "I think it really helps—not with muscle but more with recovery after an exhausting set," he said.

"It's like a spaceship. It's very interesting technology."

The pod, which is seven feet long, three feet wide and seven feet high with the lid open, looks like a cross between a tanning bed and the giant egg Lady Gaga emerged from at the Grammys.

CVAC says its pod is different from other pressurized chambers on the market because it combines altitude pressure with cyclic compression (a combination some studies suggest is more effective than one or the other). Because the pressure, temperature and air density in the CVAC pod can be adjusted, the company says it enhances an athlete's ability to adapt to a range of conditions.

Click here to read more on this story from the Wall Street Journal.