The developers of Novak Djokovic's 'Iron Man' equipment reveal the 'greatest secret of the Serb's career.'

Novak Djokovic's 'Iron Man' equipment may return to the US Open in the coming days. 

Eye-test and analytic fans agree in 2023. Riley has improved defensively after a terrible April. 

The 23-time Grand Slam winner called the unobtrusive chest patch's benefits 'the biggest secret of his career', and its developers have subsequently revealed them.

 Djokovic wore the coin-shaped device during the French Open and Wimbledon this year. 

 The 36-year-old remarked, "When I was a kid I liked Iron Man a lot, so I try to impersonate him," when first asked.

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