After initially struggling with the cold, the 25-year-old ran a solid final bend to blast to victory in 43.98 seconds.
After initially struggling with the cold, the 25-year-old ran a solid final bend to blast to victory in 43.98 seconds with a lot to spare, and went straight into recovery ahead of Wednesday's 200m semi-finals, with the final on Thursday.
"It was quite freezing and I struggled to get myself warmed up and ready," said Van Niekerk, who came into the 400m as defending world and Olympic champion.
"I was doubting my momentum. In the last 150 metres I tried putting in an extra gear, but I couldn’t catch my stride until my last few metres. I just allowed the race to go through to the finish line."
Thoughts will turn straight to the 200m, and a rare double.
The last athlete to claim the 200/400m double was American Michael Johnson, who achieved the feat the 1995 worlds in Gothenburg, repeating the achievement a year later in the 1996 Olympics in Atlanta.
"It's easier said than done," said Van Niekerk, who smashed Johnson's 400m world record when winning gold in Rio and then his rarely-run 300m record in Ostrava last month.
"It's competition, it's very unpredictable."
"My body still feels very good. It took me a while to recover."
"But from endurance I go straight to speed... It's a day-by-day, step-by-step process for us athletes."
Van Niekerk reiterated that every season threw up new challenges.
"I know it’s never going to be a walk in the park," he said. "I'm just so grateful to say I came through with a gold medal."
"Every year has its new challenges, and every year it gets tough. I don't think it ever became easier. Right after Rio I found out I had a back injury, and this entire season I have been struggling to find fitness, but at the same time my times have been getting better, especially in the short sprints."
Van Niekerk, the first athlete to break 10 seconds over 100m, 20sec over 200m and 44sec over 400m, added he was delighted his coach Anna "Tannie Ans" Botha would also receive a medal as part of a championships initiative to reward coaches.
"Everyone knows the superstar coach I have," he said of the 74-year-old great-grandmother who oversees the hottest property in world athletics.
"It's actually a massive honour for me to be able to reach these great heights with her."
"She has had to wait a long time – with the third gold, she gets to take one home with her too. Unfortunately my two previous medals had to go to Mum as she said 'Everything achieved at home has to stay here'. So I am glad to be able to take this one home myself!"