Liverpool's Shaun Caldwell, 27, teaches physics by day at the city's Sino-British College then mitts up at night to hit people his own age in the face and ribs. Photo provided to chinadaily.com.cn

White-collars, black eyes

Beijing's fight club

Video: White collar boxing China

When the fighters first come to Golden Gloves, the gym Benis runs near People's Square, they must sign a waiver indemnifying the organizers. It also recommends they take out insurance.

But Xiong says the worst injury he's seen since the brawls began in winter 2008 is one broken nose – and that came during a sparring session not on fight night.

In fact, since WCB first took root two decades ago there have been no deaths or serious injuries recorded.

"British people just like to complain," winces Benis. "Rugby is far more injury-prone."

Chang's case highlights the less-obvious risk: Overtraining to become fight-ready in such a short space of time for one of the most physically challenging sports in the world. It got to the point where his coaches had to tell him to stop coming to the gym every day. Fighters must attend at least three sessions a week for the WCB event.

"Make your guard wider," bellowed Shanghainese coach Raymond Wen, concerned about the 28-year-old's exposed temples amid an onslaught of strikes from sparring partner Micaelli, who boasts pinpoint accuracy and explosive speed. "Keep moving your head."

"I just couldn't figure out how to punch properly from that new guard," Chang says later. His expression suggested he was already mapping out that night's homework: More Roy Jones Jr. and Floyd Mayweather clips on YouTube. "I shadow box at home as much as I can," he adds.

"You can get hurt from any sport," he adds. "When I used to play basketball I'd sometimes get concussion, and that's the worst thing that could happen here, I guess. I've picked up a few bruises, but that was mostly when I wasn't listening to the coaches' advice."