Scott Marsden, aged 15, was competing for an English title in Leeds on Saturday when the fight had to be stopped as he required immediate attention.
The youngster, who attends Forge Valley School, in Stannington, was rushed to hospital, where it is understood he remained on Monday in a serious condition.
A message from Scott's club, Marsden's All Styles (MAS) Kickboxing, which meets at Hillsborough Arena, said training had been cancelled until further notice.
A post from the club yesterday said: "All our prayers and thoughts go out to the Marsden family."
It said the title fight had been a close one with just one point separating the fighters and nothing to suggest the contest needed to be stopped up to that point.
A spokesman for the World Kickboxing Association (WKA) England said it had been a tragic incident involving two of the country's best young fighters.
He said it was not known at this stage what had caused Scott's condition to suddenly deteriorate.
"We're communicating with Scott's family and making sure they have all the support they need. We're also giving them the time and space they require," he added.
The title bout took place at Leeds Martial Arts College in Morley.
Scott is a highly-rated young kickboxer and the club had previously raised money through crowdfunding to ensure he could represent his country at the World Kickboxing and Karate Union (WKU) World Championships in Spain.
The crowdfunding page told how Scott, then aged 13, had in the space of three months swept to victory in the WKA Yorkshire Open Championships, the WKKC National Open Championships and the WTKA European Open Championships.
Forge Valley School said it would not be appropriate to comment at this stage, out of respect to Scott's family.