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

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.

The company claims that spending up to 20 minutes in the pod three times a week can boost athletic performance by improving circulation, boosting oxygen-rich red-blood cells, removing lactic acid and possibly even stimulating mitochondrial biogenesis and stem-cell production.

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.