Allyson Felix completed a hat-trick of women's 200 metres world titles on Friday but fellow American Jeremy Wariner's bid to match her in the men's 400 failed when he lost again to compatriot LaShawn Merritt.
Smooth-striding Felix triumphed in 22.02 seconds to beat double Olympic champion Veronica Campbell-Brown of Jamaica while Merritt's success completed a personal double after he beat Wariner having already taken his Olympic title last year.
The victories took the US back above Jamaica to the top of the medals table with six golds but the night turned sour for them when their men's 4x100 relay team was disqualified.
Russia's Yaroslav Rybakov won the men's high jump, Australian Dani Samuels the women's discus while Sergey Kirdyapkin won the 50km walk to give Russia a walking clean sweep after they also took the men's and women's 20k golds.
Felix, twice an Olympic silver medallist, delivered a smooth run on a track sodden by a thunderstorm.
"I've worked so hard and I've defended my title -- I cannot ask for more," she said.
Campbell-Brown was second in 22.35 ahead of Debbie Ferguson-McKenzie of the Bahamas (22.41).
DOUBLY DEPOSED
While Felix still reigns, Wariner has been doubly deposed as the one-lap king by his one-time lieutenant.
Merritt emerged from Wariner's shadow in Beijing when he won the Olympic title and he ran a similarly patient race on Friday, coming strong in the last 100 after Wariner had gone out hard.
Merritt finished in 44.06 seconds to the 44.60 of his rival, with Rennie Quow of Trinidad & Tobago third in 45.02.
"It was all about putting it from my head to the track as I had already won the race in my heart," Merritt said. "At 350 I knew I had won."
In the sprint relay Olympic champions and world record holders Jamaica, without Usain Bolt or Asafa Powell, qualified second in their heat behind Italy.
The United States, without Tyson Gay, won their heat impressively in 37.97 seconds, having dropped the baton in the Olympic semi-finals.
However the US, who also messed up in the 1995, 1997 and 2005 worlds, were then disqualified for a changeover outside the box. A US appeal against the decision was rejected.
Rybakov won an exciting high jump final on the countback after the top four all cleared 2.32 metres. Kyriakos Ioannou took a rare silver for Cyprus while Sylwester Bednarek of Poland and Germany's Raul Spank shared bronze.
SURPRISE WINNER
Samuels, 21, was a surprise discus winner as she pulled out a big personal best 65.44-metre effort in the fifth round to overhaul Cuba's Yarelis Barrios (65.31) and veteran Romanian Nicoleta Grasu (65.20).
Yusuf Saad Kamel, Bahrain's newly-crowned 1,500 champion, qualified fastest (1:45.01) for Sunday's 800 final.
It was a bad race for Sudan though as Abubaker Kaki, fastest over the distance in the world this year, tripped early in his semi-final and failed to progress while Olympic silver medallist Ismail Ahmed Ismail dropped out at the bell in his heat.
Olympic and defending champion Maryam Yusuf Jamal was fastest into the women's 1,500 final but Olympic silver medallist Iryna Lishchynska dropped out during her semi.
Bolt continued to expand his reputation as not only the most impressive but the most popular man in the sport as he took time out on his 23rd birthday to sign hundreds of autographs for delighted fans in a 40-minute stadium walkabout.
He was then on the receiving end of a chorus of "Happy Birthday" from the crowd after receiving his 200 metres gold.
Both sprint relays are the highlight of Saturday's programme, along with the finals of the women's 5,000 and hammer and men's pole vault and long jump.
