American Sydney McLaughlin broke her own world record in the 400m hurdles in Tokyo on Wednesday.
Fellow American Dalilah Muhammad led for most of the race, but McLaughlin sped ahead at the final hurdle to take gold with a time of 51.46 seconds.
"I'm absolutely delighted. What a great race. I'm just grateful to be out here celebrating that extraordinary race and representing my country," McLaughlin said.
"I saw Dalilah ahead of me with one to go. I just thought 'Run your race'," she said.
"The race doesn't really start till hurdle seven. I just wanted to go out there and give it everything I had. It's just about trusting your training, trusting your coach, and that will get your all the way round the track."
Muhammad set a personal best time of 51.58, to take silver.
"This means everything to me at this moment," she said.
"It shows the hard work, that I've been able to accomplish it, how proud I am of this moment and putting things into perspective of the career that I've had. To be 31 and running at your very, very best is absolutely amazing."
Femke Bol, of the Netherlands, set an European record with a time of 52.03 to take the bronze.
Earlier, German javelin thrower Johannes Vetter saved his best throw for last, qualifying for the finals on his third throw in Tokyo.
Vetter sat in sixth in his group after two throws, with his best falling below qualifying at 82.08m.
His third and final throw saw him crack the mark, throwing 85.64m.
Rio 2016 silver medallist Julius Yego of Kenya, and Rio bronze medallist Keshorn Walcott of Trinidad and Tobago both failed to make the finals.
Germany's Johannes Vetter competes in the Men's Javelin Throw Qualification. Photo: Michael Kappeler/dpa
Elsewhere on the field, the first of seven events in the women's heptathlon began, with American Kendell Williams the early leader.
She ran the 100m hurdles in 12.97 seconds, ahead of Burkino Faso's Marthe Koala and Anouk Vetter of the Netherlands.
Canada's Damian Warner made his mark in the opening events of the men's decathlon.
Warner equalled his world decathlon best time in the 100 metres, crossing the line in 10.12 seconds.
“It was a great start, a start I was looking for," he said. "It is kind of on par with what I had earlier this year, when I had my lifetime best performance.
“I just have to keep building on this, and keep executing. We have high jump and 400m later today. I have to finish strong, and come out tomorrow with the same kind of energy.”
Australia's Ashley Moloney was next fastest with a time of 10.34, while fellow Canadian Pierce Lepage crossed in 10.43.
Warner, the Rio bronze medallist, also set an Olympic decathlon best in the long jump, leaping to 8.24m, beating Germany's Frank Busemann mark from 1996.
The men will compete in shot put, high jump and the 400m later Wednesday with their 110m hurdles, discus, pole vault, javelin throw and 1,500m on Thursday.
Competitors earn points for their performance in each discipline, with the gold medal going to the athlete who accrues the most points.