Joshua Cheptegei produced an astonishing run in Monaco to break the 16-year-old 5,000m world record by almost two seconds.
The 23-year-old Ugandam, who won the 10,000m world title in Doha last year, had promised he would take a shot at the time but success seemed unlikely. However, guided by trackside lights illustrating world record pace, he came home in 12 minutes 35.36 seconds.
The previous mark, set by Ethiopian great Kenenisa Bekele, was 12:37.35.
Remarkably it is Cheptegei's second world record in Monaco this year, despite the season being badly disrupted by the coronavirus pandemic. He broke the 5km road world record in the principality in February..
"Monaco is a special place and it's one of these places where I could break the world record," he said. "It took a lot to keep being motivated this year because so many people are staying at home but you have to stay motivated."
Elsewhere, world champion Noah Lyles showed his quality in the 200m with a commanding victory in 19.72 seconds. His younger brother Josephus was second on his Diamond League debut ahead of fourth-placed Adam Gemili of Britain in 20.68 seconds. Katarina Johnson-Thompson, who won heptathlon world gold last year, admitted she was "not in the best shape" after finishing sixth in the high jump with 1.84m, 14cm short of her personal best.
Cheptegei was not the only athlete to make light of the lack of competitive action in 2020 to post impressive times. Britain's Laura Muir broke Dame Kelly Holmes' 21-year-old British 1,000m record with a time of 2:30.82, in a race won by Kenya's Olympic champion Faith Kipyegon in the second fastest time in history.
"Racing a time like today gives me a lot of confidence going into an Olympic year," said Muir. "To do that in my second run, to run a British record I'm really, really pleased with it."
Norwegian world champion Karsten Warholm turned the 400m hurdles into a solo time trial, breaking Kevin Young's 1992 meeting record in 47.10 and serving notice of his intention to do the same to the American's long-standing world mark.
Scotland's Jake Wightman took more than two seconds off his 1500m personal best, coming home in 3:29.47. It earned him a creditable third behind Timothy Cheruiyot and Jakob Ingebrigtsen, world and European champion respectively, and moved him up to second in the British all-time standings.
Kenyan world champion Hellen Obiri took a comfortable win in the 5,000m, with rival Sifan Hassan stepping out of the race with a couple of laps to go and Britain's Laura Weightman claiming a personal best in third.
Great Britain's Andrew Pozzi has re-started the year in electric form. The 28-year-old, who came within three hundredths of his personal best in Finland on Tuesday, duly matched it with a time of 13.14 seconds to claim a narrow second place behind Spain's Rio 2016 silver medallist Orlando Ortega in the 110m hurdles.
Compatriot Kyle Langford earned a 800m personal best of 1:44.83 in fifth behind world champion Donavan Brazier of the United States. Previous attempts to stage international athletics this season, notably in Oslo and Zurich, have featured athletes competing remotely via video link, rarely-run distances and small fields.
With a limit of 5,000 socially-distanced spectators and star-studded start lists competing in the same stadium, Monaco's Diamond League opener was a partial return to normality. But the effects of the pandemic were still visible.
Athletes prepared for races on the infield to give them more space than Stade Louis II's regular call-room affords, crowd noise was pumped into the venue and American pole vault world champion Sam Kendricks was unable to compete after his pole failed to make it to the stadium on time.