The 25-year-old's task of landing both eased considerably with main rival Isaac Makwala of Botswana barred from running.
The 25-year-old's task of landing both eased considerably with main rival Isaac Makwala of Botswana barred from running by the sport's governing body because he had been diagnosed with the highly contagious norovirus.
It didn't prevent the 30-year-old from turning up at the warm-up track only to be turned away. The International Association of Athletics Federations (IAAF) said he should be in quarantine and not near the track.
"It was definitely a heartbreaking moment," Van Niekerk said. "I have a lot of sympathy for Isaac."
"It is quite disappointing. I would have liked him to have his opportunity."
"But this is sports, these things happen."
Van Niekerk added that the double was not a given.
"It's easier said than done," he said. "It's competition, it's very unpredictable.
"My body still feels very good. It took me a while to recover tonight."
"But from endurance I go straight to speed tomorrow... It's a day-by-day, step-by-step process for us athletes."
Botswana suffered further disappointment as 800m race favourite Nijel Amos -- second on the same track in the 2012 Olympic final -- faded to finish fifth with Frenchman Pierre-Ambroise Bosse taking a deserved gold for a bold and brave performance which he revealed afterwards reflected his love of gambling.
"I am a gambler, I love going to the casino," said Bosse.
"And today I just gambled, I put everything on the red, even my last Euro. So hopefully, this is also for luck in love."
However, for his compatriot Renaud Lavillenie his world gold drought continued even in the stadium where he won Olympic gold in 2012.
The Frenchman had to settle for bronze -- his fifth minor medal in the world championships -- with American Sam Kendricks taking the title in front of his parents and girlfriend to boot.
Conseslus Kipruto maintained Kenya's fine record in the men's 3000 metres steeplechase winning in cheeky style from Morocco's Soufiane El bakkali with long-time leader Evan Jager of the United States third.
Kipruto appeared to be under pressure from Elbakkali going to the last obstacle but once over it the Olympic champion cupped his hand to his ear and pounded his chest.
"I used my plans well and last night for morale I told myself: 'I am Olympic champion and that others must break me'," said Kipruto.
"There are others who are strong but I used my own plans. I knew if the race was around 8:10 that I was going to win."
There was a second world title for the Czech Republic's women's javelin world record holder Barbora Spotakova.
The 36-year-old -- a two-time Olympic champion -- won with a mark of 66.76 metres.
"At this stadium, I am unbeatable," said Spotakova.
"There must be something in the air about London. I cannot explain it but when I enter this stadium, I always feel so calm and relaxed."
"The whole day I was thinking about my last world title which was ten years ago in Osaka. Actually, it is also almost exactly five years since my Olympic title here in London. These facts made me feel very emotional."
There could also be a women's double in the 200 and 400m as Shaunae Miller-Uibo, the Bahamas 400m Olympic champion, looked good in her 200m heat -- she runs in the 400m final on Wednesday -- and with her chances boosted as America's 100m gold medallist Tori Bowie did not start because of the after effects of her tumble after crossing the line in Sunday's victory.