Great Britain's Keely Hodgkinson fell just short of the 800m world title in a dazzling home-straight duel with American Olympic champion Athing Mu.
Hodgkinson, who admitted she was "a bit gutted" with silver, finished in 1:56.38, eight hundredths off Mu.
The pair went toe-to-toe down the final 100m as Hodgkinson pulled alongside the home crowd favourite, but could not quite pass her for victory.
"I gave it everything right up to the line," Hodgkinson said.
"A world silver is not too bad. It adds fuel to the fire. I will go away and I still have more championships to come so will have to refocus for that.
"I'm gunning for a bit more so I am looking forward to next year."
Hodgkinson's rivalry with Mu, also 20, seems set to define the event for the World Championships in Budapest in 2023, the Olympics in Paris in 2024 and, potentially, many years beyond.
Both barely broke a smile as they were introduced to the crowd before the start, the tension of their contest clear.
They hit the bell in a swift 57.09 seconds, and the race boiled down to the expected face-off down the back straight.
Mu made the first move, stretching her legs to open a lead, but Hodgkinson's counter followed soon after.
The Briton dragged herself back alongside Mu and seemed poised to pass up the inside in a dramatic heist.
But Mu found just enough to hold on to gold and hold her rival at bay.
As she collected her breath after the finish line, Hodgkinson motioned to her team in the stands, her finger and thumb set a couple of inches apart to show the fine margin of defeat.
The gap to Mu is small. And, as coach Trevor Painter and Jenny Meadows will remind her, diminishing.
Ingebrigtsen bounces back from 1500m defeat
Jakob Ingebrigtsen, beaten by Britain's Jake Wightman in his preferred 1500m earlier in the week, bounced back with a superb victory in the 5000m.
The Norwegian had suggested he would end his season in the immediate aftermath of his defeat by Wightman, but changed his mind and was rewarded with his first world title as he strode clear of Kenya's Jacob Krop and Uganda's Oscar Chelimo.
Joshua Cheptegei, who won 5000m Olympic gold in Tokyo last summer, finished down in ninth with Great Britain's Marc Scott 14th.
Nigeria's Tobi Amusan, who ran a world record in the 100m semi-finals earlier in the day, took gold as she blitzed away from Jamaica's Britany Anderson and Puerto Rico's Jasmine Camacho-Quinn.
Amusan's winning time of 12.06 seconds would have been an improvement on the 12.12 second world record she had set a couple of hours before, but was ruled ineligible by a tailwind over the legal limit.
Great Britain's Cindy Sember came fifth in 12.38 seconds, a time that would have further lowered the national record she set in the semi-finals.
Britons Jazmin Sawyers and Lorraine Ugen finished ninth and 10th in the long jump with efforts of 6.62m and 6.53m respectively. Germany's Malaika Mihambo, who fouled on her first two jumps, defended her world title, leaping out to 7.12m.
Armand Duplantis was in his class of his own in the pole vault. The Swedish world record-holder made a first-time clearance of 6.06m to clinch gold.
Nobody else in the field managed to clear higher than 5.94m.
