South African track and field athlete Wayde van Niekerk has won the 400m at the 2017 IAAF World Championships in London, and is now focusing on the 200m.

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.

"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."

"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."

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