With resources limited and some coaches absent, Xu and his team were forced to improvise in the gym.

"We had 10 boxers at that time but just one coach, so I would be accompanied by other boxers when I trained," explained Xu.

"They would point out my weaknesses, and in return I would also share my experience when I acted as the assistant."

After returning to China, Xu and his team were quarantined for 14 days in Yuxi, Yunnan province, where he continued to train in his hotel room as best he could.

"Some basic routines such as skipping, push-ups, and sit-ups didn't need that much space, I was able to complete them in my room," said Xu.

"Training is an inseparable part of my life, just like the meals."

It's been a whirlwind last few years for Xu, who hails from Fuzhou, Jiangxi province. In January 2019, he became China's first WBA champion by dethroning Jesus Rojas of Puerto Rico by unanimous decision in Houston, Texas.

Xu retained his WBA belt in November by throwing an incredible 1,562 punches (landing 402 of them) during an absorbing contest against American challenger Manny Robles III via unanimous decision in California.

Xu knows he will have to live up to his 'Monster' nickname again to upset the odds against Warrington, who says he would reluctantly agree to a fight behind closed doors.