Marco Arop goes from back to front to take 800m gold
Normally a front runner, he kept his huge frame out of the picture, right at the back, through the first lap and while the rest of the field were wondering, he swept straight to the front at the bell and surged clear. Arop kept his foot down to win in 1:44.24.
Kenya's Emmanuel Wanyonyi (19) took silver in 1:44.53 while Ben Pattison (21) continued Britain's great middle distance form at the championships with bronze in 1:44.83.
"This means everything to me. I'm so glad to finally be able to do this on the world stage," said Arop, who was born in Sudan and forced by the civil war to flee with his family of eight when he was two years old.
"After bronze in Eugene I just concentrated on making progress, step by step, each year trying to become better than I was the year before. Now it's going to be hard to beat a gold medal - I'll just have to repeat it, I guess."
Addressing his change of tactics, he added: "I don't want to be known as the guy who can only run one way. I tried to be patient tonight and put myself in a position where I could attack. I knew it was going to be a tactical race.
"I was up late last night figuring different ways to win. One scenario was to kick from behind in the way I did. I guess it was worth staying up late."
It was an impressive performance by Pattison, who belied his lack of top-level experience by clinging on determinedly to the inside of the inside lane and finding enough acceleration coming off the final bend to stay clear of the chasing pack.
"Before the race my coach told me I could race to win or race for a medal. I said I wanted to win," said Pattison, who was the second-slowest coming into the nine-man final field.
"I don't care if I come fourth or eighth, no-one remembers that. Top three is what it's all about and I've just managed that
"One of the last people I saw today before I got on the bus was (new 1500m champion and fellow Briton) Josh Kerr. Before I could even say well done to him, he said well done to me.
"This is just crazy, that a great champion like him comes to me like that. This whole experience is really insane."
Disaster for US' 4x400 relay team
The United States women's 4x400 metres relay team, who have won seven of the last eight World Championships and the last seven Olympic golds, were disqualified from Saturday's semi-finals after a botched baton changeover.
The US were trailing Britain after three legs when Alexis Holmes initially failed to collect the baton from Quanera Hayes and by the time she got hold of it she appeared to have gone beyond the "blue box".
The US recovered to finish second behind the British in the heat but were marked as disqualified. They are likely to appeal.
Jamaica and Canada were the fastest into Sunday's final.
The similarly dominant US' men, who have won eight of the last nine world titles and four of the last five Olympics, also had a stutter as they were pushed all the way in the first heat, however by India.
Justin Robinson brought them home in 2:58.47 but only after fighting off a final bend challenge by Rajesh Ramesh that helped his team to an Asian record 2:59.05 - the first time they have cracked three minutes.
Britain snatched the third automatic place from Botswana by eight thousandths of a second but the Africans went through comfortably as their fastest loser time was quicker than the winners of the second semi.
Jamaica, perennial global silver medallists in recent years, won the second heat in 2:59.82, with fast-finishing France and Italy following them through. Fifth-placed Belgium, bronze medallists in the last two worlds, did not make it.