“She told us she was going to go to Rio and do this,” mum Jasmin revealed. “I never doubted that she would.”
Cox was 23 when she was diagnosed with multiple sclerosis, shortly after having a stroke. Now she is the first British Paralympian for 28 years to win medals in two sports at the same Games.
“That day in hospital, she couldn’t walk,” said Jasmin, in Rio with husband Asmond and their other children, Joah and Kamisha.
“But when she gets knocked down, she always gets up. She rises to whatever challenge faces her.
“The first time it was the stroke, then the MS, but she pushed herself and got back up. Now look at her.”
In the space of 24 hours, Cox won 100m bronze at the athletics track and smashed the 500m time trial world record to win cycling gold.
She still has two more athletics events and a road race on the bike to come.
If she gives the impression of being a woman in a hurry, that’s because she is.
“I have relapsing remitting MS and I don’t know how much more time as an athlete I have,” said Cox, who finds running easier than walking.
“By the time Tokyo 2020 comes along, I might not even be in a position to take part in one event.”
She focuses on the here and now – and in so doing hopes more than anything to inspire others battling adversity.
“Initially, it was a journey about one girl who wanted to go to the Paralympics,” she explained. “But over the two years it has become something that I was doing for everyone else.
“That’s the reason I wanted to do this so badly; so that I could stand here and show it can be done, even if you have setbacks.
“That’s why I’m so emotional – because it’s for everyone else around the world. My message is ‘never give up’.”
Her words were echoed by team-mate Rachel Morris, another Briton to reach the top in two different para sports.
A hand-cycling gold medalist in Beijing eight years ago, Morris was hit by a car while out training in 2012 and decided it was safer on the water.
On Sunday, just three years after learning to row, she took one of three British gold medals inside an hour at Lagoa by winning the single sculls.
“Yes, it’s personal,” she said. “But it’s also about giving people a way of believing that they can achieve something.”
On what fast became Super Sunday, there were also golds for Lauren Rowles and Laurence Whiteley in the mixed double sculls and the mixed coxed four.